Adds an else condition to the array builder in `validate`
Adds an else condition to the array builder in `validate`

- Instead of wrapping the whole validation routine inside a check for
  whether the $_POST key was set, we add an else condition for what
  to do if all the other paths return false. This was preventing us
  from deleting the metadata if the field had been cleared.

cfpb/cms-toolkit,kurtw/cms-toolkit,Scotchester/cms-toolkit,kurtw/cms-toolkit,Scotchester/cms-toolkit,cfpb/cms-toolkit