Fix media display for no media_data row in sql.

Fix media display for no media_data row in sql.

If there is no media_data row for the current media (for
whatever reason, there might be good ones), let
MediaEntry.media_data not raise an exception but just
return None.

The exif display part now handles this by checking whether
.media_data.exif is defined (None has no attribute exif, so
it's undefined, all fine).