changeset can be None in getTreeEntries

Seen in HGitaly logs while investigating another problem

Traceback (most recent call last):
  File "/home/hg/venv/lib/python3.9/site-packages/grpc/_server.py", line 461, in _take_response_from_response_iterator
    return next(response_iterator), True
  File "/home/hg/venv/lib/python3.9/site-packages/hgitaly/service/commit.py", line 323, in GetTreeEntries
    sha = changeset.hex().decode('ascii')
AttributeError: 'NoneType' object has no attribute 'hex'

(no other information easily found, but this can probably be protected anyway)

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