Improve SkeletonRenderer for scripting

[pandamicro]

If we want to remove the texture: In C++, we can pass nullptr, but in lua or js, passing null won't be accepted by the binding function. What we can do is suggested in the picture, when receiving empty string, just use nullptr instead.

[pandamicro]

It's fixed by accepting null in JS and convert to nullptr by binding functions.
Lua should support this too, @samuele3hu please check

[pandamicro]

@dualface Please check whether lua null can be passed as parameter and be converted to nullptr in C++