Fix an issue where blame could fatal for unrecognized authors

Summary: See PHI255. See https://discourse.phabricator-community.org/t/error-generating-blame-data/766.

Test Plan:

• Viewed a file contributed to by users with no Phabricator user accounts, in Diffusion.
• Enabled blame.
• Before patch: blame failed, fatal in logs.
• After patch: blame worked.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18833