Fix applypatch_check failure when applying update on angler

Applypatch_check should be skipped if no sha is specified. As the
comments said: "It's okay to specify no sha1s; the check will pass if
the LoadFileContents is successful. Useful for reading partitions,
where the filename encodes the sha1s."

Test: The update package applied on angler successfully.
Bug: 32243751
Change-Id: Ib8f3dadf19f745c2dbd350d60da46ab12d75bc87