Allow lease expiry to be +ve when the corresponding blobs expiry is 0.

It is fine for an app to aquire a lease until a certain time
even if the corresponding the blob doesn't have a expiry date associated
with it.

Fixes: 149789374
Test: atest --test-mapping apex/blobstore
Change-Id: I76f7bdb616d78a5d83a8489a898f1e6015589f38