Bug 782974

This bug is created as a clone of upstream ticket:
https://fedorahosted.org/freeipa/ticket/2159

Trying to erase the contents of a config option will return various kinds of errors.

These are not marked as Required so we don't have to pass each and every one on each mod but they cannot be empty.

{{{
$ ipa config-mod --groupsearch=

ipa: DEBUG: raw: config_mod(ipagroupsearchfields=None, rights=False, all=False, raw=False, version=u'2.17')
ipa: DEBUG: config_mod(ipagroupsearchfields=None, rights=False, all=False, raw=False, version=u'2.17')
ipa: ERROR: non-public: AttributeError: 'NoneType' object has no attribute 'split'
Traceback (most recent call last):
  File "/home/rcrit/redhat/freeipa-master/ipaserver/rpcserver.py", line 223, in wsgi_execute
    result = self.Command[name](*args, **options)
  File "/home/rcrit/redhat/freeipa-master/ipalib/frontend.py", line 438, in __call__
    ret = self.run(*args, **options)
  File "/home/rcrit/redhat/freeipa-master/ipalib/frontend.py", line 696, in run
    return self.execute(*args, **options)
  File "/home/rcrit/redhat/freeipa-master/ipalib/plugins/baseldap.py", line 1118, in execute
    ldap, dn, entry_attrs, attrs_list, *keys, **options
  File "/home/rcrit/redhat/freeipa-master/ipalib/plugins/config.py", line 221, in pre_callback
    fields = entry_attrs[k].split(',')
AttributeError: 'NoneType' object has no attribute 'split'
ipa: INFO: admin: config_mod(ipagroupsearchfields=None, rights=False, all=False, raw=False, version=u'2.17'): AttributeError
ipa: DEBUG: response: InternalError: an internal error has occurred
ipa: DEBUG: Destroyed connection context.ldap2
}}}