Fixed ERR_INVALID_PARAMETER in animation.gd

[endaye]

Fixed an error in animation.gd.

Problem

• If animation_finished signal is already connected to Dialogic.Animation.animation_finished, it will cause this error @GlobalScope.ERR_INVALID_PARAMETER

Solution

• According to Signal docs, use is_connected first to check for existing connections.

[Jowan-Spooner]

Hey thank you for the PR! I didn't think this would be necessary due to the ONE_SHOT connections (so it should disconnect before it comes up the next time).
But if you encountered the error message and this fixes it, while maintaining the same functionality, that is great. I'll test it in a moment and merge after.

[Jowan-Spooner]

Looks good to me! Thanks.

[endaye]

Yes. When you connect a ONE_SHOT signal twice before you emit it, it will return the error.