ipaautomountmap: Allows clearing description attribute with "".

This change allows clearing automountmap 'description' attribute by
passing an empty string ("") as the playbook parameter.

New test cases were added to check this behavior.

plugins/modules/ipaautomountmap.py

@@ -126,7 +126,8 @@ class AutomountMap(IPAAnsibleModule):
         _args = {}
         if mapname:
             _args["automountmapname"] = mapname
-        if desc:
+        # An empty string is valid and will clear the attribute.
+        if desc is not None:
             _args["description"] = desc
         return _args