Fix exception when encoding is not defined

Summary:
the code tries to access 'encoding' property even when the
repository is empty. The fix is to set it to null in that case.

Test Plan: run the conduit method on my sandbox and it works now.

Reviewers: grglr, epriestley, nh

Reviewed By: grglr

CC: aran, grglr

Differential Revision: 1075