Fix bot_update parsing of gerrit footers with empty values.

Follow up based on https://chromium-review.googlesource.com/487963

R=agable@chromium.org, machenbach@chromium.org, maruel@chromium.org

Bug: 715614,717504
Change-Id: I0902647c8fa8a2ff9643890c029fc8b75a72ff7e
Reviewed-on: https://chromium-review.googlesource.com/493466
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>

recipes/recipe_modules/bot_update/resources/bot_update.py

@@ -47,7 +47,7 @@ BRANCH_HEADS_REFSPEC = '+refs/branch-heads/*'
 TAGS_REFSPEC = '+refs/tags/*'
 
 # Regular expression that matches a single commit footer line.
-COMMIT_FOOTER_ENTRY_RE = re.compile(r'([^:]+):\s+(.+)')
+COMMIT_FOOTER_ENTRY_RE = re.compile(r'([^:]+):\s*(.*)')
 
 # Footer metadata keys for regular and gsubtreed mirrored commit positions.
 COMMIT_POSITION_FOOTER_KEY = 'Cr-Commit-Position'