Docs: Avoid use of 'default: null' (#45795)

Various modules document the default 'null' value, but it causes None to
be shown in the documentation explicitly.

lib/ansible/modules/network/routeros/routeros_command.py

@@ -1,20 +1,7 @@
 #!/usr/bin/python
-#
-# This file is part of Ansible
-#
-# Ansible is free software: you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation, either version 3 of the License, or
-# (at your option) any later version.
-#
-# Ansible is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-#
-# You should have received a copy of the GNU General Public License
-# along with Ansible.  If not, see <http://www.gnu.org/licenses/>.
-#
+
+# GNU General Public License v3.0+ (see COPYING or https://www.gnu.org/licenses/gpl-3.0.txt)
+
 from __future__ import (absolute_import, division, print_function)
 __metaclass__ = type
 
@@ -50,8 +37,6 @@ options:
         before moving forward. If the conditional is not true
         within the configured number of retries, the task fails.
         See examples.
-    required: false
-    default: null
   match:
     description:
       - The I(match) argument is used in conjunction with the
@@ -60,7 +45,6 @@ options:
         then all conditionals in the wait_for must be satisfied.  If
         the value is set to C(any) then only one of the values must be
         satisfied.
-    required: false
     default: all
     choices: ['any', 'all']
   retries:
@@ -69,7 +53,6 @@ options:
         before it is considered failed. The command is run on the
         target device every retry and evaluated against the
         I(wait_for) conditions.
-    required: false
     default: 10
   interval:
     description:
@@ -77,7 +60,6 @@ options:
         of the command. If the command does not pass the specified
         conditions, the interval indicates how long to wait before
         trying the command again.
-    required: false
     default: 1
 """