Fixes #7912. Make sure .cur() only returns 0 as fallback value when it needs to ("", auto, undefined, null).

This change makes .cur() more .cssHooks friendly. .cur() now returns the unmodified value by
.css() if it isn't a number, number-alike or a value that needs a fallback to 0.
This way fx.start doesn't need to be recalculated for complex values.