Don't show ok button in AddBuildingType if category value is null

[Helium314]

Currently when selecting e.g. "for cars" category in AddBuildingType, the ok button is shown, but when clicking ok the user is asked to select a more specific value.
With this change, the ok button is not shown any more when selecting such a category. This should avoid unnecessary attempts on selecting some categories, while at the same time providing a subtle hint that categories can actually be used for answering the quest too.

[mnalis]

If I understand correctly, it would only apply to For cars, On a farm and Other categories, while other categories (Residential, Commercial building, Civic building, Religious building) would still retain OK button and be selected? Good idea!

Also, if OK button is not shown in such cases, would it make quest_generic_item_invalid_value text and related code unneeded?

[Helium314]

If I understand correctly, it would only apply to For cars, On a farm and Other categories, while other categories [...] would still retain OK button and be selected?

Exactly

Also, if OK button is not shown in such cases, would it make quest_generic_item_invalid_value text and related code unneeded?

I don't think so. It's been a while, but I think it is/was somehow possible to press the ok button when it shouldn't be visible at all. Possibly related to the appear/disappear animation.
Though maybe the button showing condition is checked again when pressing the ok button, didn't look this up.

[westnordost]

Well, whether or not that string could be removed now (and the code that makes it show) would be something for a different PR. Thanks @Helium314