Fix Physics Picking captured Object initialization

[Sauermann]

Initialize variables when a captured object is known.

Without this initialization, the following lines will fail, resulting in some events not being sent to the collision object:

godot/scene/main/viewport.cpp

Lines 846 to 847 in a83eb16

	if (last_id.is_valid()) {
	if (ObjectDB::get_instance(last_id) && last_object) {

MRP: godotengine/godot-demo-projects#925

1. Drag one of the sliders.
2. Drop while moving the mouse (sometimes the Mouse-button-up event is not registered)

[RandomShaper]

It's a bit hard to assess for me. I trust the changes enough to be put in the wild, but I can't confidently give them greenlight for 4.1.

[YuriSizov]

Then let's merge it early in 4.2. And if it proves to be a safe fix, we can cherry-pick it.

[YuriSizov]

Thanks!