Filter CP listing by blank or boolean field values

[jacksleight]

I'd like to be able to find entries in the control panel where a specific field is blank. This is useful to check if data is missing from entries.

I've implemented this with a custom filter for now, which works fine but it's more or less a copy of the existing Fields filter, it'd be great if there was a built-in "Is Empty" option in this dropdown:

A similar option for searching boolean values would also be useful. You can do is: 1 to find true values, but this isn't obvious to regular users, and you can't do is: 0 to find false values.

Happy to PR if there's interest.