Bug 782566

This bug is created as a clone of upstream ticket:
https://fedorahosted.org/freeipa/ticket/2255

Modifying permission memberof to empty will generate an internal error.

See the following permission:
{{{
# ipa permission-add test --permission=all --memberof=editors --type=user
-----------------------
Added permission "test"
-----------------------
  Permission name: test
  Permissions: all
  Type: user
  Member of group: editors
}}}

Modifying the memberof to another value works fine:
{{{
# ipa permission-mod test --memberof=ipausers
--------------------------
Modified permission "test"
--------------------------
  Permission name: test
  Permissions: all
  Type: user
  Member of group: ipausers
}}}

Modifying memberof to empty fails:
{{{
# ipa permission-mod test --memberof=
ipa: ERROR: an internal error has occurred
}}}

Here is the stack trace from /var/log/httpd/error_log:
{{{
ipa: ERROR: non-public: AttributeError: 'NoneType' object has no attribute 'find'
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/ipaserver/rpcserver.py", line 229, in wsgi_execute
    result = self.Command[name](*args, **options)
  File "/usr/lib/python2.7/site-packages/ipalib/frontend.py", line 438, in __call__
    ret = self.run(*args, **options)
  File "/usr/lib/python2.7/site-packages/ipalib/frontend.py", line 696, in run
    return self.execute(*args, **options)
  File "/usr/lib/python2.7/site-packages/ipalib/plugins/baseldap.py", line 1106, in execute
    ldap, dn, entry_attrs, attrs_list, *keys, **options
  File "/usr/lib/python2.7/site-packages/ipalib/plugins/permission.py", line 299, in pre_callback
    raise e
AttributeError: 'NoneType' object has no attribute 'find'
ipa: INFO: admin: permission_mod(u'test', memberof=None, rights=False, all=False, raw=False, version=u'2.20'): AttributeError
}}}