These default assignements have not actually been needed/useful since a
very long time already. It was likely a left-over from early stages of
the project. While not a problem so far, it makes it harder to
distinguish RNA properties whith a custom liboverride handling, from the
ones using the default code.
Keeping them to `nullptr` by default also makes these callbacks definition
behavior more in line with other RNA properties callbacks, which are
usually non-null only when a custom function is defined.
No behavioral changes are expected from this commit.
aa9599119a