* Support Is (not) null for all multi criterions
Add support for the Is null and Is not null modifiers for all cases of
the MultiCriterionInput and HierarchicalMultiCriterionInput. This
partially overlaps the "X Count" filter which sometimes is available
(because it would be the same as "X Count equals 0" and "X Count greater
than 0") but this also enables it for other criterions like the "Parent
Studio" filter for studios or just the "Studios" filter for scenes /
images / galleries, the "Movies" filter for scenes etc.
* Don't crash UI on bad saved filter
* Add missing code for tag parent/child
Co-authored-by: WithoutPants <53250216+WithoutPants@users.noreply.github.com>
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