[1.6.x] Fixed #21750 -- Fixed regression introduced by 4befb301.

Validating STATIC_ROOT in StaticFilesStorage.__init__ turned out to be
problematic - especially with tests - because the storage refuses to work even
if there are no actual interactions with the file system, which is backward
incompatible.

Originally the validation happened in the StaticFilesStorage.path method, but
that didn't work as expected because the call to FileSystemStorage.__init__
replaced the empty value by a valid path. The new approach is to move back the
check to the StaticFilesStorage.path method, but ensure that the location
attribute remains None after the call to super.

Refs #21581.

Backport of 1e9e7351 from master