trace2: prevent segfault on config collection where no value specified

When TRACE2 analytics is enabled, a git config option that has no value
causes a segfault.

Steps to Reproduce
GIT_TRACE2=true GIT_TRACE2_CONFIG_PARAMS=status.*
git -c status.relativePaths version
Expected Result
git version 2.46.0
Actual Result
zsh: segmentation fault GIT_TRACE2=true

This adds checks to prevent the segfault and instead return
an empty value.

Signed-off-by: Adam Murray <ad@canva.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: ad-murray

t/t0210-trace2-normal.sh

@@ -243,6 +243,14 @@ test_expect_success 'bug messages followed by BUG() are written to trace2' '
 	test_cmp expect actual
 '
 
+test_expect_success 'empty configuration values are handled' '
+  test_when_finished "rm trace2.normal actual expect" &&
+  echo >expect &&
+  GIT_TRACE2="$(pwd)/trace2.normal"  GIT_TRACE2_CONFIG_PARAMS=foo.empty \
+	  git -c foo.empty config foo.empty >actual &&
+	test_cmp expect actual
+'
+
 sane_unset GIT_TRACE2_BRIEF
 
 # Now test without environment variables and get all Trace2 settings

trace2.c

@@ -764,7 +764,7 @@ void trace2_def_param_fl(const char *file, int line, const char *param,
 	if (!trace2_enabled)
 		return;
 
-	redacted = redact_arg(value);
+	redacted = value ? redact_arg(value): NULL;
 
 	for_each_wanted_builtin (j, tgt_j)
 		if (tgt_j->pfn_param_fl)

trace2/tr2_tgt_event.c

@@ -493,7 +493,8 @@ static void fn_param_fl(const char *file, int line, const char *param,
 	event_fmt_prepare(event_name, file, line, NULL, &jw);
 	jw_object_string(&jw, "scope", scope_name);
 	jw_object_string(&jw, "param", param);
-	jw_object_string(&jw, "value", value);
+	if (value)
+		jw_object_string(&jw, "value", value);
 	jw_end(&jw);
 
 	tr2_dst_write_line(&tr2dst_event, &jw.json);

trace2/tr2_tgt_normal.c

@@ -307,8 +307,9 @@ static void fn_param_fl(const char *file, int line, const char *param,
 	enum config_scope scope = kvi->scope;
 	const char *scope_name = config_scope_name(scope);
 
-	strbuf_addf(&buf_payload, "def_param scope:%s %s=%s", scope_name, param,
-		    value);
+	strbuf_addf(&buf_payload, "def_param scope:%s %s", scope_name, param);
+	if (value)
+		strbuf_addf(&buf_payload, "=%s", value);
 	normal_io_write_fl(file, line, &buf_payload);
 	strbuf_release(&buf_payload);
 }

trace2/tr2_tgt_perf.c

@@ -448,8 +448,9 @@ static void fn_param_fl(const char *file, int line, const char *param,
 	struct strbuf scope_payload = STRBUF_INIT;
 	enum config_scope scope = kvi->scope;
 	const char *scope_name = config_scope_name(scope);
-
-	strbuf_addf(&buf_payload, "%s:%s", param, value);
+	strbuf_addstr(&buf_payload, param);
+	if (value)
+		strbuf_addf(&buf_payload, ":%s", value);
 	strbuf_addf(&scope_payload, "%s:%s", "scope", scope_name);
 
 	perf_io_write_fl(file, line, event_name, NULL, NULL, NULL,