ipagroup: Fix ensuring external group group members (without trust-ad)

Due to an API misbehaviour in FreeIPA, ipaexternalmembers need to be
treated differently than other group members parameters. Even an empty
array triggers all tests for external members, including the check for
installed dcerpc bindings.

Therefore ipagroup module has been changed to not set ipaexternalmember
to an empty list if there are no external members to be added or
removed.