validate_failures.py: Fix only_for_subset option

Change-Id: Ide7776e01bd548c89a2995fef2f06beada98a99a
diff --git a/contrib/testsuite-management/validate_failures.py b/contrib/testsuite-management/validate_failures.py
index 4afbfbe..5dbeae1 100755
--- a/contrib/testsuite-management/validate_failures.py
+++ b/contrib/testsuite-management/validate_failures.py
@@ -741,7 +741,9 @@
   manifest = GetManifest(manifest_path)
   sum_files = GetSumFiles(_OPTIONS.results, _OPTIONS.build_dir)
   actual = GetResults(sum_files)
-  subset = GetResults([ _OPTIONS.only_for_subset ])
+  subset = None
+  if _OPTIONS.only_for_subset:
+    subset = GetResults([ _OPTIONS.only_for_subset ])
 
   if _OPTIONS.verbosity >= 5:
     print('\n\nTests expected to fail')