Do not convert resource dirs without resources

Previously we would convert to java_resources when there were no
resources and get an error.

Test: b build //external/libphonenumber/... --config=android
Change-Id: I86f30b993a3ffdb20000f6a4c5261e2c4c74263c

bp2build/java_library_conversion_test.go

@@ -441,12 +441,13 @@ func TestJavaLibraryResourcesExcludeFile(t *testing.T) {
 func TestJavaLibraryResourcesWithMultipleDirs(t *testing.T) {
 	runJavaLibraryTestCase(t, Bp2buildTestCase{
 		Filesystem: map[string]string{
-			"res/a.res":  "",
-			"res1/b.res": "",
+			"res/a.res":   "",
+			"res1/b.res":  "",
+			"res2/b.java": "",
 		},
 		Blueprint: `java_library {
 	name: "java-lib-1",
-	java_resource_dirs: ["res", "res1"],
+	java_resource_dirs: ["res", "res1", "res2"],
 	sdk_version: "current",
 }`,
 		ExpectedBazelTargets: []string{