Potentially inconsistent List behavior for insert_after and insert_before for when an element doesn't exist

[Xrayez]

Godot version:
6f4384f

Issue description:
I know this may be silly and not sure whether this could be a copy-paste mistake or indented, and I'm not that experienced with using std::list either to know the expected behavior.

If the element doesn't exist in the list (nullptr), then insert_after constructs a new element and pushes it back to the list

godot/core/list.h

Lines 279 to 284 in 6f4384f

	Element *insert_after(Element *p_element, const T &p_value) {
	CRASH_COND(p_element && (!_data || p_element->data != _data));
	if (!p_element) {
	return push_back(p_value);
	}

But then if we take the same situation but use insert_before, it does the same:

godot/core/list.h

Lines 305 to 310 in 6f4384f

	Element *insert_before(Element *p_element, const T &p_value) {
	CRASH_COND(p_element && (!_data || p_element->data != _data));
	if (!p_element) {
	return push_back(p_value);
	}

Expected result:
The insert_before should push_front() if the target element doesn't exist in the list, and not push_back.

The other question is that whether this behavior should be allowed in the first place, and simply error-out on attempting to insert a new element before or after an element which doesn't exists.

Steps to reproduce:
Some GUT pseudo-code:

func test_insert_after_null():
	populate_test_data(list)
	list.insert_after(null, "Godot")
	assert_eq(list.back.value, "Godot") # OK.


func test_insert_before_null():
	populate_test_data(list)
	list.insert_before(null, "Godot")
	assert_eq(list.front.value, "Godot") # Fails here.

Minimal reproduction project:
Not minimal but the context this was found: goostengine/goost#12.