Replace the Option + expect("checked above") pattern in both upload
short-circuit paths with a filtered if-let binding, so the presence of
the blob key is guaranteed structurally instead of by a comment on a
non-local invariant. Also collapse the longhand match on
existing_write_key into `?`. No behavior change.
Signed-off-by: Brad Seiler <seiler@squareup.com>
4622480662