In commit a96d3cc3f (cache-tree: reject entries with null sha1,
2017-04-21), the git.git project has changed its stance on null OIDs in
tree objects. Previously, null OIDs were accepted in tree entries to
help tools repair broken history. This resulted in some problems though
in that many code paths mistakenly passed null OIDs to be added to a
tree, which was not properly detected.

Align our own code base according to the upstream change and reject
writing tree entries early when the OID is all-zero.