GDScript: Don't use the NIL address to hold return value of functions

[vnen]

This prevents that the NIL address is filled with another value, which causes problems for some instructions that read from NIL.

This is still a bit of a flimsy solution, we need to find a way to disambiguate the NIL target (i.e. discard the return value) and the NIL source (i.e. use a Variant of type NIL). I thought of some ways to do it but any of those would take a while to implement and test, so it's best if done later.

Fix #70964
Fix #70936
Fix #70953

[vnen]

Added a fix to #70953 which is also a regression introduced in my other PR.

[akien-mga]

Added a fix to #70953 which is also a regression introduced in my other PR.

From that PR, you also have #71065 to handle I believe.

[akien-mga]

Thanks!