Gd::from_variant(&Variant::nil()) causes crash

[atrefonas]

This follows a discussion we were having in the Discord.

Gd::from_variant(&Variant::nil()) causes a hard crash.

My backtrace from the crash is:

(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x68)
  * frame #0: 0x000000010405a2a0 godot.macos.editor.arm64`___lldb_unnamed_symbol144153
    frame #1: 0x0000000113a22670 libeco_game.dylib`godot_core::obj::gd::Gd$LT$T$GT$::from_opaque::h855c84a7a86296cb(opaque=(storage = "", marker = core::marker::PhantomData<unsigned char *> @ 0x000000016fdfc410)) at gd.rs:219:27
    frame #2: 0x00000001139bdf60 libeco_game.dylib`_$LT$godot_core..obj..gd..Gd$LT$T$GT$$u20$as$u20$godot_ffi..godot_ffi..GodotFfi$GT$::from_sys_init::hbe5869979e1f6c59(init={closure_env#0}<godot_core::gen::classes::kinematic_collision_3d::re_export::KinematicCollision3D> @ 0x000000016fdfc4b0) at godot_ffi.rs:149:13
    frame #3: 0x00000001139bbe88 libeco_game.dylib`_$LT$godot_core..obj..gd..Gd$LT$T$GT$$u20$as$u20$godot_core..builtin..variant..variant_traits..FromVariant$GT$::try_from_variant::ha7b25a7a4b6e343e(variant=0x000000016fdfcb48) at gd.rs:595:13
    frame #4: 0x00000001139bbe3c libeco_game.dylib`godot_core::builtin::variant::variant_traits::FromVariant::from_variant::h47e14a615036a1f6(variant=0x000000016fdfcb48) at variant_traits.rs:19:9
    frame #5: 0x00000001139ddc74 libeco_game.dylib`eco_game::main_scene_3d::Main3D::on_ray_process::h1ef5fa7f2599b46d(self=0x000000011108f708) at main_scene_3d.rs:112:11

The reason this is important is so that null default arguments can be passed into functions in GDExtension. For example in the original GDScript API, PhysicsBody3D.test_move() has a null default argument for the 3rd argument.

An ugly workaround that reportedly works is unsafe { mem::transmute((ptr::null::<Node>(), None::<InstanceId>)) }, although I have not tried it yet with the PhysicsBody3D api.

Also, ideally it seems the 3rd argument of PhysicsBody3D.test_move() should be an Option<Gd> not just a Gd, so there is also a codegen issue here, which maybe is the bigger issue actually.

[Bromeon]

Thanks! Ideally, Gd::try_from_variant(&Variant::nil()) should return Err(VariantConversionError), and from_variant() thus panic.

This goes a bit in a similar direction as #77, although the two are not directly related.