internal/sarif: use empty arrays instead of nils

Sarif specification requires that some slice elements explicitly exist
in the JSON output even if they are empty. For instance, results should
be an empty array if the sarif handler finished but found nothing.
Another example is tags. Each rule in govulncheck sarif has tags
property that can sometimes be empty. If so, JSON should contain an
empty slice of tags.

Fixes golang/go#70157

Change-Id: I112181e4efa5bc0a1577ff98f1b9eab912ed814c
Reviewed-on: https://go-review.googlesource.com/c/vuln/+/625656
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Maceo Thompson <maceothompson@google.com>

cmd/govulncheck/testdata/common/testfiles/source-call/source_call_sarif.ct

@@ -496,3 +496,33 @@ $ govulncheck -C ${moddir}/vuln -format sarif ./...
     }
   ]
 }
+
+# Test sarif json output for no vulnerabilities
+$ govulncheck -C ${moddir}/novuln -format sarif ./...
+{
+  "version": "2.1.0",
+  "$schema": "https://json.schemastore.org/sarif-2.1.0.json",
+  "runs": [
+    {
+      "tool": {
+        "driver": {
+          "name": "govulncheck",
+          "semanticVersion": "v0.0.0",
+          "informationUri": "https://pkg.go.dev/golang.org/x/vuln/cmd/govulncheck",
+          "properties": {
+            "protocol_version": "v1.0.0",
+            "scanner_name": "govulncheck",
+            "scanner_version": "v0.0.0-00000000000-20000101010101",
+            "db": "testdata/vulndb-v1",
+            "db_last_modified": "2023-04-03T15:57:51Z",
+            "go_version": "go1.18",
+            "scan_level": "symbol",
+            "scan_mode": "source"
+          },
+          "rules": []
+        }
+      },
+      "results": []
+    }
+  ]
+}

cmd/govulncheck/testdata/common/testfiles/source-call/source_call_text.ct

@@ -135,3 +135,7 @@ Your code is affected by 2 vulnerabilities from 1 module.
 This scan also found 1 vulnerability in packages you import and 1 vulnerability
 in modules you require, but your code doesn't appear to call these
 vulnerabilities.
+
+# Test no vulnerabilities in source mode
+$ govulncheck -C ${moddir}/novuln ./...
+No vulnerabilities found.

internal/sarif/handler.go

@@ -142,7 +142,7 @@ func toSarif(h *handler) Log {
 }
 
 func rules(h *handler) []Rule {
-	var rs []Rule
+	rs := make([]Rule, 0, len(h.findings)) // must not be nil
 	for id := range h.findings {
 		osv := h.osvs[id]
 		// s is either summary if it exists, or details
@@ -157,15 +157,24 @@ func rules(h *handler) []Rule {
 			FullDescription:  Description{Text: s},
 			HelpURI:          fmt.Sprintf("https://pkg.go.dev/vuln/%s", osv.ID),
 			Help:             Description{Text: osv.Details},
-			Properties:       RuleTags{Tags: osv.Aliases},
+			Properties:       RuleTags{Tags: tags(osv)},
 		})
 	}
 	sort.SliceStable(rs, func(i, j int) bool { return rs[i].ID < rs[j].ID })
 	return rs
 }
 
+// tags returns an slice of zero or
+// more aliases of o.
+func tags(o *osv.Entry) []string {
+	if len(o.Aliases) > 0 {
+		return o.Aliases
+	}
+	return []string{} // must not be nil
+}
+
 func results(h *handler) []Result {
-	var results []Result
+	results := make([]Result, 0, len(h.findings)) // must not be nil
 	for osv, fs := range h.findings {
 		var locs []Location
 		if h.cfg.ScanMode != govulncheck.ScanModeBinary {
@@ -283,7 +292,7 @@ func stack(h *handler, f *govulncheck.Finding) Stack {
 	trace := f.Trace
 	top := trace[len(trace)-1] // belongs to top level module
 
-	var frames []Frame
+	frames := make([]Frame, 0, len(trace)) // must not be nil
 	for i := len(trace) - 1; i >= 0; i-- { // vulnerable symbol is at the top frame
 		frame := trace[i]
 		pos := govulncheck.Position{Line: 1, Column: 1}
@@ -350,7 +359,7 @@ func codeFlows(h *handler, fs []*govulncheck.Finding) []CodeFlow {
 }
 
 func threadFlows(h *handler, fs []*govulncheck.Finding) []ThreadFlow {
-	var tfs []ThreadFlow
+	tfs := make([]ThreadFlow, 0, len(fs)) // must not be nil
 	for _, f := range fs {
 		trace := traces.Compact(f)
 		top := trace[len(trace)-1] // belongs to top level module

internal/sarif/sarif.go

@@ -66,7 +66,7 @@ type Run struct {
 	Tool Tool `json:"tool,omitempty"`
 	// Results contain govulncheck findings. There should be exactly one
 	// Result per a detected use of an OSV.
-	Results []Result `json:"results,omitempty"`
+	Results []Result `json:"results"`
 }
 
 // Tool captures information about govulncheck analysis that was run.
@@ -85,7 +85,7 @@ type Driver struct {
 	// Properties are govulncheck run metadata, such as vuln db, Go version, etc.
 	Properties govulncheck.Config `json:"properties,omitempty"`
 
-	Rules []Rule `json:"rules,omitempty"`
+	Rules []Rule `json:"rules"`
 }
 
 // Rule corresponds to the static analysis rule/analyzer that
@@ -105,7 +105,7 @@ type Rule struct {
 
 // RuleTags defines properties.tags.
 type RuleTags struct {
-	Tags []string `json:"tags,omitempty"`
+	Tags []string `json:"tags"`
 }
 
 // Description is a text in its raw or markdown form.
@@ -143,7 +143,7 @@ type Result struct {
 // for, say, a particular symbol or package.
 type CodeFlow struct {
 	// ThreadFlows is effectively a set of related information flows.
-	ThreadFlows []ThreadFlow `json:"threadFlows,omitempty"`
+	ThreadFlows []ThreadFlow `json:"threadFlows"`
 	Message     Description  `json:"message,omitempty"`
 }
 
@@ -165,7 +165,7 @@ type ThreadFlowLocation struct {
 // Stack is a sequence of frames and can encode a govulncheck call stack.
 type Stack struct {
 	Message Description `json:"message,omitempty"`
-	Frames  []Frame     `json:"frames,omitempty"`
+	Frames  []Frame     `json:"frames"`
 }
 
 // Frame is effectively a module location. It can also contain thread and