ipauser: Refactor module due to fix on arguments comparison.

Due to a change in 'ansible_freeipa_module.compare_args_ipa', playbook
parameters using empty strings are correctly evaluated, and do not need
to be removed before comparison is performed.

A new test playbook, with tests for clearing attributes with an empty
string ("") is available at:

    tests/user/test_user_empty_lists.yml