Ensure that `?author=0` resolves to blog user

[mattwiebe]

When we recently introduced the move to ?author=user_id style Actor URLs, we got caught on a check that failed on ?author=0 IDs for the Blog User.

Fixes #981

Proposed changes:

• Always just return from the regex that parses the author URL. The regex has already confirmed there's a number there.

Other information:

• [ ] Have you written new tests for your changes, if applicable?

Testing instructions:

• Go to '..'