Make sure offset is an integer

Summary:
Older git versions allowed the --skip to get empty string as value and interpreted that to an int.
We were sending null in some places where we wanted a 0, so it all worked out.

Since https://git.kernel.org/pub/scm/git/git.git/commit/revision.c?id=71a1e94821666909b7b2bd62a36244c601f8430e git fails when provided empty string for an integer.

See Q124 and T15783.

Test Plan: Have recent git version (that fails with --skip '', navigate to /diffusion/1/history/master/) - see history.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: aklapper, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25575