For operators with dynamic tooltips the tooltip calculation is
deferred until the moment it is actually shown for performance
reasons, with the tooltip field left blank for the time being.
Enum menu code shouldn't jump in and assign a tooltip either.
The menu button itself can't show a dynamic tooltip because it
does not actually call the operator, and has no reference to it.
As a side change, allow returning None from the python callback
as the most natural way to fall back to the default tooltip.
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