Ensure that date-calculated fields always have an int64 value. Fixes #23

I hit a problem with import -- still unresolved -- that caused repos to be brought in with null last_commit and last_pull dates. last_pull being null seems like a valid thing to me, though last_commit probably wouldn't be under normal circumstances.

Anyway, in that case, the calcs for the last_pull and last_commit values were returning null because the fields didn't have values, and that was causing the UI to be empty, due to the scan error described in #23 .  This ensures that even in the aberrant case of null values, the UI still works as expected