Missing null for First go to frame in Animation

[CedricGuillemet]

Follow up https://forum.babylonjs.com/t/issue-animation-gotoframe/29493/3

_frameToSyncFromJump for was null and was not used correctly for 1st go to frame. After the 1st go to frame, it's set to null and everything is fine.

[azure-pipelines]

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[azure-pipelines]

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