Credential Manager: Accept empty URLs.

LocalCredential and FederatedCredental should accept empty avatar URLs.

BUG=400674

Review URL: https://codereview.chromium.org/649573002

git-svn-id: svn://svn.chromium.org/blink/trunk@183527 bbb929c8-8fbe-4397-9dbb-9b2b20218538