soong_zip after dex should not fail when no *.dex files were generated

This can happen if Android application has only a manifest file and no
java code.

Fixes: 124483155
Test: treehugger with Android.mk converted to Android.bp in
platforms/frameworks/base/tests/privapp-permnissions

Change-Id: I45f95680b62a7a4bda0e7ab156f3df24b0dad70b
diff --git a/java/dex.go b/java/dex.go
index 913eee6..54b7bfc 100644
--- a/java/dex.go
+++ b/java/dex.go
@@ -177,9 +177,9 @@
 	javalibJar := android.PathForModuleOut(ctx, "dex", jarName)
 	outDir := android.PathForModuleOut(ctx, "dex")
 
-	zipFlags := ""
+	zipFlags := "--ignore_missing_files"
 	if j.deviceProperties.UncompressDex {
-		zipFlags = "-L 0"
+		zipFlags += " -L 0"
 	}
 
 	if useR8 {