Allow passing a null callback to xsprintf

Summary: This is not very useful in general, but makes sense in the context of D11661.

Test Plan: See D11661.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: richardvanvelzen, Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11679