Legacy screenshots were broken due to some assumptions made in the new

image diff code. They checked for an attribute to be non-null, and if

that was the case, it would activate diffs, which then required certain

state not available for screenshots. Instead, we just want to check that

the attribute has a truthy value.
This is a pretty simple change, and results in both screenshots and

image file attachment (with and without diffs) working correctly once

again.

Tested with legacy screenshots, and image file attachments (with different

diff ranges, and without diffs).