Adding tests for cel expressions demonstrating use of has

I was reading through this test to get a handle for the type of expressions I could
use in overlays and filter expressions and noticed that there weren't any tests
demonstrating the capabilities discussed in #890.

This change adds two tests within the existing structure of the cel_test to help
this file be used as a place to go for example expressions that should work or,
alternative, fail.

pkg/interceptors/cel/cel_test.go

@@ -230,6 +230,19 @@ func TestInterceptor_Process(t *testing.T) {
 				"other": "thing",
 			},
 		},
+	}, {
+		name: "demonstrate defaulting logic within cel interceptor",
+		CEL: &triggersv1.CELInterceptor{
+			Overlays: []triggersv1.CELOverlay{
+				{Key: "one", Expression: "has(body.value) ? body.value : 'default'"},
+				{Key: "two", Expression: "has(body.test.first) ? body.test.first : 'default'"},
+			},
+		},
+		body: json.RawMessage(`{"value":"test","test":{"other":"thing"}}`),
+		wantExtensions: map[string]interface{}{
+			"one": "test",
+			"two": "default",
+		},
 	}, {
 		name: "filters and overlays can access passed in extensions",
 		CEL: &triggersv1.CELInterceptor{
@@ -696,6 +709,11 @@ func TestExpressionEvaluation_Error(t *testing.T) {
 			expr: "body.value.marshalJSON()",
 			want: "unexpected type 'string' passed to marshalJSON",
 		},
+		{
+			name: "has function missing nested key",
+			expr: "has(body.pull_request.repository.owner) ? body.pull_request.repository.owner : 'me'",
+			want: `failed to evaluate: no such key: repository`,
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(rt *testing.T) {