Description of problem: ipa trustconfig-show throws internal error.

Version-Release number of selected component (if applicable):
ipa-server-trust-ad-4.4.0-1.el7.x86_64
ipa-server-4.4.0-1.el7.x86_64

How reproducible: Always

Steps to Reproduce:
1. Install ipa-server
2. run ipa-adtrust-install
3. run ipa trustconfig-show
4. Check the message displayed on the console.

Actual results:
[root@server ~]# ipa trustconfig-show
ipa: ERROR: non-public: TypeError: trustconfig_show.validate_output():
  output['value']: need (<type 'NoneType'>,); got <type 'unicode'>: u'ad'
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/ipalib/backend.py", line 137, in
execute
    result = self.Command[_name](*args, **options)
  File "/usr/lib/python2.7/site-packages/ipalib/frontend.py", line 447, in
__call__
    return self.__do_call(*args, **options)
  File "/usr/lib/python2.7/site-packages/ipalib/frontend.py", line 486, in
__do_call
    self.validate_output(ret, options['version'])
  File "/usr/lib/python2.7/site-packages/ipalib/frontend.py", line 993, in
validate_output
    nice, o.name, o.type, type(value), value)
TypeError: trustconfig_show.validate_output():
  output['value']: need (<type 'NoneType'>,); got <type 'unicode'>: u'ad'
ipa: ERROR: an internal error has occurred


Expected results:
Proper error message should be displayed.

Additional info: