module_utils: Fix comparison of elements not in IPA object.

This change modifies the comparison of the retrieved IPA object and the
provided arguments on ansible_freeipa_module.compare_args_ipa when the
provider argument is an empty string.

If an attribute is not available in 'ipa', its value is considered to be
a list with an empty string (['']), possibly forcing the conversion of
the 'args' attribute to a list for comparison. This allows, for example,
the usage of empty strings which should compare as equals to inexistent
attributes (None), as is done in IPA API.