Fixes some AnimationPlayer bugs

Fixes #25745 (closed) Fixes #17423 (closed)

Line 1210 from animation_player.cpp is: https://github.com/godotengine/godot/blob/eda4be193fd736d362184bc937efc99c1faa18c3/scene/animation/animation_player.cpp#L1210

Which means that when called without arguments the method would replace the variable playback.assigned with an empty String. This makes the signal animation_finished give this empty String as an argument.

I've changed it to https://github.com/godotengine/godot/blob/592d514380eabfc9d36c68b44f263513e823c35b/scene/animation/animation_player.cpp#L1210

The variable name in this method has the correct animation name as defined at the beginning of it. This fixes #25745 (closed).

The line 1208: https://github.com/godotengine/godot/blob/eda4be193fd736d362184bc937efc99c1faa18c3/scene/animation/animation_player.cpp#L1208

Doesn't have any condition to run, so it resets the position of the animation to 0 every time p_from_end is false. Also, calling stop(true) doesn't reset the animation. This PR makes the animation position be 0 when calling stop(true) https://github.com/godotengine/godot/blob/592d514380eabfc9d36c68b44f263513e823c35b/scene/animation/animation_player.cpp#L1281-L1294

and replaces line 1208 with https://github.com/godotengine/godot/blob/592d514380eabfc9d36c68b44f263513e823c35b/scene/animation/animation_player.cpp#L1208

The animation position is reset, or not, at the stop() method, so the play() method just keeps the decision made there (unless it is told to run the animation backwards), fixing #17423 (closed).