[release] Fix parsing of gerrit footers with empty values.

This ports:
https://chromium-review.googlesource.com/c/493466/

The code was originally copied from bot_update. So were the bugs.

Bug: chromium:717504
NOTRY=true
TBR=tandrii@chromium.org,agable@chromium.org

Change-Id: If2d2dafdca8cd44f325dc770dfc42c17889a3b4a
Reviewed-on: https://chromium-review.googlesource.com/501787Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45224}

tools/release/git_recipes.py

@@ -32,7 +32,7 @@ SHA1_RE = re.compile('^[a-fA-F0-9]{40}$')
 ROLL_DEPS_GIT_SVN_ID_RE = re.compile('^git-svn-id: .*@([0-9]+) .*$')
 
 # 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 key for commit position.
 COMMIT_POSITION_FOOTER_KEY = 'Cr-Commit-Position'