Allow Control to show custom tooltip when tooltip text is empty

[timothyqiu]

Fixes #97944
Fixes #97990

In #97397, I changed the implementation of button's shortcut_in_tooltip from overriding get_tooltip() to overriding make_custom_tooltip(). However, if a button only has shortcut and no actual tooltip text, then the viewport assumes that the tooltip is not needed and make_custom_tooltip() is no longer called.

This PR makes the tooltip displayed when the tooltip text is not empty or make_custom_tooltip() returns a valid control.

This is useful when you want to make a customized tooltip without meaningful tooltip text.

[bruvzg]

Tested with #97944 and #97990, all tooltips seem to be working as expected.

[Repiteo]

Thanks!