Avoid nil-pointer panic when Rego template source is missing

constraint/pkg/client/drivers/rego/schema/schema.go

@@ -54,6 +54,10 @@ func (in *Source) ToUnstructured() map[string]interface{} {
 // GetSource extracts Source from a templates.Code object.
 func GetSource(code templates.Code) (*Source, error) {
 	rawCode := code.Source
+	if rawCode == nil || rawCode.Value == nil {
+		return nil, fmt.Errorf("%w: source", ErrMissingField)
+	}
+
 	v, ok := rawCode.Value.(map[string]interface{})
 	if !ok {
 		return nil, ErrBadType

constraint/pkg/client/drivers/rego/schema/schema_test.go

@@ -1,6 +1,7 @@
 package schema
 
 import (
+	"errors"
 	"testing"
 
 	"github.com/open-policy-agent/frameworks/constraint/pkg/core/templates"
@@ -77,3 +78,25 @@ func TestGetSourceVersions(t *testing.T) {
 		})
 	}
 }
+
+func TestGetSourceMissingSource(t *testing.T) {
+	testCases := map[string]templates.Code{
+		"nil source": {
+			Engine: Name,
+			Source: nil,
+		},
+		"nil source value": {
+			Engine: Name,
+			Source: &templates.Anything{},
+		},
+	}
+
+	for name, code := range testCases {
+		t.Run(name, func(t *testing.T) {
+			_, err := GetSource(code)
+			if !errors.Is(err, ErrMissingField) {
+				t.Fatalf("expected %v, got %v", ErrMissingField, err)
+			}
+		})
+	}
+}