[vm] Fix presubmit script to only invoke clang-tidy if there are any .h/.cc files

Change-Id: I180392e5f77f6a5fa81c529b109e35c76f4deef6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/115367
Reviewed-by: Dmitry Stefantsov <dmitryas@google.com>
Reviewed-by: Jens Johansen <jensj@google.com>

PRESUBMIT.py

@@ -232,6 +232,9 @@ def _CheckClangTidy(input_api, output_api):
         path = f.LocalPath()
         if is_cpp_file(path): files.append(path)
 
+    if not files:
+        return []
+
     args = [
         './buildtools/linux-x64/clang/bin/clang-tidy',
         '-checks=readability-implicit-bool-conversion',