Fix for "cannot marshal None unless.." (#33660)

when using only an activation key without any channels.
As already suggested by mattclay in
https://github.com/ansible/ansible/pull/25079

and also patch unit test for rhn_register and
add test case for activationkey only

lib/ansible/modules/packaging/os/rhn_register.py

@@ -292,6 +292,8 @@ class Rhn(redhat.RegistrationBase):
         os.unlink(self.config['systemIdPath'])
 
     def subscribe(self, channels):
+        if not channels:
+            return
         if self._is_hosted():
             current_channels = self.api('channel.software.listSystemChannels', self.systemid)
             new_channels = [item['channel_label'] for item in current_channels]

test/units/modules/packaging/os/test_rhn_register.py

@@ -110,9 +110,9 @@ def test_without_required_parameters(capfd, patch_rhn):
 TESTED_MODULE = rhn_register.__name__
 TEST_CASES = [
     [
-        # Registering an unregistered host
+        # Registering an unregistered host and subscribing to one channel
         {
-            'activationkey': 'key',
+            'channels': 'rhel-x86_64-server-6',
             'username': 'user',
             'password': 'pass',
         },
@@ -136,6 +136,26 @@ TEST_CASES = [
             'unlink.call_count': 0,
         }
     ],
+    [
+        # Registering an unregistered host with only an activationkey and without subscribing any channels
+        {
+            'activationkey': 'key',
+        },
+        {
+            'calls': [
+            ],
+            'is_registered': False,
+            'is_registered.call_count': 1,
+            'enable.call_count': 1,
+            'systemid.call_count': 0,
+            'changed': True,
+            'msg': "System successfully registered to 'rhn.redhat.com'.",
+            'run_command.call_count': 1,
+            'run_command.call_args': '/usr/sbin/rhnreg_ks',
+            'request_called': False,
+            'unlink.call_count': 0,
+        }
+    ],
     [
         # Register an host already registered, check that result is unchanged
         {