### Godot version

4.0.beta13

### System information

Windows 10

### Issue description

When you call the `seek()` function without a current animation it outputs `Condition "!playback.current.from" is true.` to the debugger.

I was getting the error above whenever I would call `stop()` on an AnimationPlayer when it didn't have a playing animation. I realized that `stop()` also calls the `seek()` function.

I would expect the `seek()` and `stop()` functions to just do nothing, instead of throwing errors, if there isn't a current animation, or at least show an error that says that there is no animation to seek/stop.

This is the same issue that was described here: [#6122](https://github.com/godotengine/godot/issues/6122)

### Steps to reproduce

- Create an AnimationPlayer node.
- Don't have it play any animations.
- Call the `seek()` function.

### Minimal reproduction project

[seek_issue.zip](https://github.com/godotengine/godot/files/10442071/seek_issue.zip)

if (!playback.current.from) {
		if (playback.assigned) {
			ERR_FAIL_COND_MSG(!animation_set.has(playback.assigned), vformat("Animation not found: %s.", playback.assigned));
			playback.current.from = &animation_set[playback.assigned];
		}
		ERR_FAIL_COND(!playback.current.from);
	}
	playback.current.pos = p_time - p_delta;
	if (speed_scale != 0.0) {
		p_delta /= speed_scale;
	}
	_animation_process(p_delta);
	
}
bool AnimationPlayer::is_valid() const {
	return (playback.current.from);
}
double AnimationPlayer::get_current_animation_position() const {
	ERR_FAIL_COND_V_MSG(!playback.current.from, 0, "AnimationPlayer has no current animation");
	return playback.current.pos;
}
double AnimationPlayer::get_current_animation_length() const {