[1.x] JIT sends 304 responses for deleted originals

[michael-e]

If an original image file has been deleted, $last_modified_gmt and $etag will be null. This will always make one of the conditions in this line return true.

Thus, JIT will send a 304 response for revalidation requests (If-None-Match resp. If-Modified-Since). Forever! Which is wrong, obviously.

I will send a PR to 1.x.