grit: Add fallback_to_default_layout attribute to <output>

grit/format/rc.py

Index: grit/format/rc.py
diff --git a/grit/format/rc.py b/grit/format/rc.py
index c766eff156ccd93fa4272021de8ed3a4320bd674..eb8efaca1d9f9217450841ef24ae9245957c4734 100644
--- a/grit/format/rc.py
+++ b/grit/format/rc.py
@@ -432,7 +432,11 @@ def FormatInclude(item, lang, output_dir, type=None, process_html=False):
   #
   # The FileForLanguage() Function has the side effect of generating the file
   # if needed (e.g. if it is an HTML file include).
-  filename = os.path.abspath(item.FileForLanguage(lang, output_dir))
+  file_for_lang = item.FileForLanguage(lang, output_dir)
+  if file_for_lang is None:
+    return ''
+
+  filename = os.path.abspath(file_for_lang)
   if process_html:
     filename = item.Process(output_dir)
   elif filename_only: