argus's review note: the emptiness guard was one-sided. A reviewed sha
of "" was already treated as unknown, but a head of "" was not — so a
forge returning an empty string rather than omitting the field would
make every review compare unequal and mark the whole PR stale. That is
the wrong-direction failure this function exists to prevent, and the
asymmetry contradicted its own doc comment.
Both sides now check emptiness, for the same reason: a blank string is
a value the forge sent, not a sha it has.
0e64f46221