Make sure initialHash exists before checking its value

(Followup on If17b50cc4a39993)
Make sure initialHash is set before checking and changing its value
in the MWImageModel method. If a user chooses a brand new image to
insert into the document (an insertion rather than update) then the
initial hash is empty, and changing its values will fail with an
error message.

Bug: 72492
Change-Id: I3f9ea74891cc0ab77a07fa5d4e9cbc591f5b93e6
Merge "Make sure initialHash exists before checking its value"