Fix sound play function not accepting zero as a valid offset

[docEdub]

The Sound.play function is not accepting zero as a valid offset. This makes resetting the offset to zero not work after previously setting it to a non-zero value.

Reported on forum here: https://forum.babylonjs.com/t/pausing-and-playing-an-audio-with-offset-restarts-it-from-the-beginning-instead-of-the-current-position/36668/12

This change fixes the issue by comparing the given offset argument to undefined instead of using JavaScript truthy/falsey.

This bug was caused by the change I made in #13373.

[bjsplat]

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[bjsplat]

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