fixes unit test about main-mock

by ensuring .cpp_linter_cache folder exists during unit test.

also don't parse empty XML data; return early instead.

Author: 2bndy5

lib/clang_format.dart

@@ -143,6 +143,7 @@ class FormatFix {
 /// Parse the [xmlOut] from running clang-format on a single [file].
 FormatFix parseFormatReplacementsXml(String xmlOut, FileObj file) {
   FormatFix advice = FormatFix(file);
+  if (xmlOut.isEmpty) return advice;
   final document = XmlDocument.parse(xmlOut);
   for (final child in document.root.findAllElements('replacement')) {
     var offset = int.parse(

test/run_test.dart

@@ -59,6 +59,7 @@ void main() async {
         .toList();
     final (ignored, notIgnored) = parseIgnoredOption(args['ignore']);
     final files = listSourceFiles(ext, ignored, notIgnored);
+    Directory('.cpp_linter_cache').createSync();
     final (formatAdvice, _, tidyNotes) = await captureClangToolsOutput(
       files,
       args['version'],