Fails on pulling from empty repo

[domanchi]

If a repository is empty, detect-secrets-server will attempt to clone it, yet fail at pulling master.

~/.detect-secrets-server/repos/c13c25fda3384f857b42705cca3c7483b745b5fdb17819578346d4091d97d9803286fcd9495540bfa6e7ee1051cede1d72a25b72d9b4b7c790e9d5ce4697c82c
$ git --git-dir `pwd` --work-tree . pull
fatal: Couldn't find remote ref HEAD

We should handle this issue gracefully.

Offending Line:

detect-secrets-server/detect_secrets_server/storage/base.py

Line 83 in db0c9e9

git.pull_master(self._repo_location)