Keep Variant type after zero()

[KoBeWi]

Fixes godotengine/godot-demo-projects#979
Regression from #80813

Variant.zero() (apparently used by animation) is supposed to revert the value to default. However in case of some types, it clears internals and changes the type to NIL. This PR ensures that the type is retained and initializes the value anew. (tested only with Transform3D)

[akien-mga]

Might be slighter better to add a bool reset_type = true to clear() instead of doing it like this?

[KoBeWi]

_clear_internal() called by clear() will free the internal data, so it needs to be re-created.
The best solution would be probably reworking zero() to not use clear().

[akien-mga]

Looks fairly safe, zero() is only used in a few paths:

$ rg -g'!thirdparty' "[\.\t]zero\("
scene/animation/animation_mixer.cpp
661:                                            track_value->init_value.zero();

core/math/basis.h
187:            rows[0].zero();
188:            rows[1].zero();
189:            rows[2].zero();

servers/physics_3d/godot_body_3d.cpp
71:                             center_of_mass_local.zero();

center_of_mass_local is Vector2, and rows in Basis are real_t, so neither will be affected.

[TokageItLab]

I was trying to get the same result by different ways of writing it, but this seems to be the simplest and better way. Also I applied this to the Array Blending I am working on and it works well.

[akien-mga]

Thanks!

[aaronfranke]

See also #59186. Since this method is supposed to reset to the default value, calling it "zero" doesn't seem correct.