Add empty tag support for java_import

Test: manual
Change-Id: I53d7c6bb620fa8fe7f5c3ec0a12f723eb62fca16
diff --git a/java/java.go b/java/java.go
index 3ce1885..a973bab 100644
--- a/java/java.go
+++ b/java/java.go
@@ -2731,7 +2731,7 @@
 
 func (j *Import) OutputFiles(tag string) (android.Paths, error) {
 	switch tag {
-	case ".jar":
+	case "", ".jar":
 		return android.Paths{j.combinedClasspathFile}, nil
 	default:
 		return nil, fmt.Errorf("unsupported module reference tag %q", tag)