Fixes AnimationGroup isPlaying when mask is applied & played more than once

[s207152]

A followup fix of #15836 and forum issue:
https://forum.babylonjs.com/t/animationgroup-isplaying-is-always-true-if-animationgroupmask-is-applied/54799

When AnimationGroupMask is applied, isPlaying will set to false on the first time, which works as expected.

However if the animation group is played more than once, isPlaying will still be true after the animation has ended (can be reproduced on the playground provided in the forum issue).

It is due to AnimationGroup._animatables.length being assumed to be 0 when the animation has ended.
So explicitly clear AnimationGroup._animatables should finally fix the issue.

[bjsplat]

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[bjsplat]

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