empty source during decoding should not be fatal (#3791)

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

syft/format/syftjson/model/source.go

@@ -58,7 +58,8 @@ func (s *Source) UnmarshalJSON(b []byte) error {
 func unpackSrcMetadata(s *Source, unpacker sourceUnpacker) error {
 	rt := sourcemetadata.ReflectTypeFromJSONName(s.Type)
 	if rt == nil {
-		return fmt.Errorf("unable to find source metadata type=%q", s.Type)
+		// in cases where we are converting from an SBOM without any source information, we don't want this to be fatal
+		return nil
 	}
 
 	val := reflect.New(rt).Interface()

syft/format/syftjson/model/source_test.go

@@ -22,6 +22,15 @@ func TestSource_UnmarshalJSON(t *testing.T) {
 		expected *Source
 		wantErr  require.ErrorAssertionFunc
 	}{
+		{
+			name: "empty",
+			input: []byte(`{
+				"id": "",
+				"type": "",
+				"metadata": null
+			}`),
+			expected: &Source{},
+		},
 		{
 			name: "directory",
 			input: []byte(`{