Fix error when stopping empty animation player.

[EAinsley]

Fix #97714

[SaracenOne]

Good catch, LGTM 👍
That said, it has made me aware that there's other edge cases we need to catch too, like what happens if you're playing an AnimationPlayer and then delete the animations while its still playing.

[EAinsley]

Good catch, LGTM 👍 That said, it has made me aware that there's other edge cases we need to catch too, like what happens if you're playing an AnimationPlayer and then delete the animations while its still playing.

That's why we need more tests for these edge cases.

[akien-mga]

Thanks!