transition() mixin prevent redundant 'transition: none' and pass `null`

Created by: ysds

This PR includes three points:

1. transition(null) and transition(none) output the following redundant CSS, but should be prevented.

@media (prefers-reduced-motion: reduce) {
  selector {
    transition: none;
  }
}

2. Fix the problem where libsass(node-sass) can't handle transition(null) well:

selector {
  @include transition(null);
}
// output
selector {
  transition:;
}

3. The transition mixin now show warning message when use none or null with multiple arguments.

Before: https://www.sassmeister.com/gist/b4d2d3ce8601116308e72df97a0d309a After: https://www.sassmeister.com/gist/7ddae4ab995e0b41bbb93abe6eff616f