Fix #19423 by defaulting to 0 instead of none

[barryp]

"transform: rotate(0);" makes sense as a default if no --fa-rotate-angle is defined.

I understand that:

• I'm submitting this PR for reference only. It shows an example of what I'd like to see changed but
  I understand that it will not be merged and I will not be listed as a contributor on this project.

[tagliala]

Hi!

Thanks for being part of the Font Awesome Community and for this PR

I think this is a legit bug, because we should go either for transform: none or transform: rotate(0), because transform: rotate(none) is not valid

For this specific use case, I think that transform: rotate(0) is the correct way to go.

/cc @robmadole