Nit fix for git_footers.

TBR=machenbach@chromium.org
Bug: 717504
Change-Id: Ifa71fa4cac0b57ed11ba5e7eb93516b23b72a496
Reviewed-on: https://chromium-review.googlesource.com/502809Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>

git_footers.py

@@ -49,7 +49,7 @@ def matches_footer_key(line, key):
   """
   r = parse_footer(line)
   if r is None:
-    return None
+    return False
   return normalize_name(r[0]) == normalize_name(key)