Fix build error from llndk.libraries.txt

llndk.libraries.txt build fails from clang tools build, as there is no
valid target path for llndk.libraries.txt from the build. This change
skips llndk.libraries.txt build if target output path is empty.

Bug: 330100430
Test: prebuilts/clang-tools/build-prebuilts.sh passed
Change-Id: I867dc90dbf98e3d8c24b191a744686286740b230
diff --git a/cc/llndk_library.go b/cc/llndk_library.go
index ae9da98..cd931f6 100644
--- a/cc/llndk_library.go
+++ b/cc/llndk_library.go
@@ -121,6 +121,11 @@
 }
 
 func (txt *llndkLibrariesTxtModule) GenerateSingletonBuildActions(ctx android.SingletonContext) {
+	if txt.outputFile.String() == "" {
+		// Skip if target file path is empty
+		return
+	}
+
 	ctx.VisitAllModules(func(m android.Module) {
 		if c, ok := m.(*Module); ok && c.VendorProperties.IsLLNDK && !c.Header() && !c.IsVndkPrebuiltLibrary() {
 			filename, err := getVndkFileName(c)