Return GIT_PASSTHROUGH if credentials callback returns nill

[brandonweeks]

Fixed #625, see libgit2/libgit2#3902 for more information. Since triggering the error condition requires connecting to a repository that requires authentication I wasn't sure what a good way to test this would be.

[carlosmn]

This doesn't look like the correct behaviour in this case. Returning nil from a credentials callback is a bug in that callback and it's completely different from indicating that the callback isn't set, including the error message which would now be misleading.