crunch_flags: handle non-string ACONFIG_VALUE_SETS

If the default value for RELEASE_ACONFIG_VALUE_SETS is not a string,
the resulting value is the empty string.

Bug: 328495189
Test: manual
Change-Id: Ie8f262d6408b1564ec7a3918f8654ccfa9bc5940

cmd/release_config/crunch_flags/main.go

@@ -137,7 +137,9 @@ func ProcessBuildFlags(dir string, namespaceMap map[string]string) error {
 		workflow := rc_proto.Workflow(rc_proto.Workflow_PREBUILT)
 		switch {
 		case declName == "RELEASE_ACONFIG_VALUE_SETS":
-			rootAconfigModule = declValue[1 : len(declValue)-1]
+			if strings.HasPrefix(declValue, "\"") {
+				rootAconfigModule = declValue[1 : len(declValue)-1]
+			}
 			continue
 		case strings.HasPrefix(declValue, "\""):
 			// String values mean that the flag workflow is (most likely) either MANUAL or PREBUILT.