[REVIEW ONLY] Fix for #6742: Set initial value for cubic-bezier

This PR for #6742 is not ready at all, but I would like to hear your opionion on that piece of code @redmunds

__Todo__:
- [x] Set values for an empty `cubic-bezier()` (according to @redmunds, we should use `ease-in-out` here)
- [x] Set values for an invalid `cubic-bezier(-5, 0.7, 8, "invalid")` (only replace the invalid ones with default values)
- [ ] Set values for an `cubic-bezier(0, 0, 0)` with an invalid number of params (only replace the missing/invalid ones with default values)
- [ ] Set values for an empty `steps()`
- [ ] Set values for an invalid `steps("invalid", 2)` (only replace the invalid ones with default values)
- [ ] Set values for an `steps(1, 2, 3)` with an invalid number of params (only replace the missing/invalid ones with default values)
- [ ] Add unit tests