Do not enforce setting msg in exit_json

In plain Ansible, `msg` is mandatory only for `fail_json`.

src/ansible.lua

@@ -406,7 +406,6 @@ function Ansible:fail_json(kwargs)
 end
 
 function Ansible:exit_json(kwargs)
-	assert(kwargs['msg'])
 	if nil == kwargs['changed'] then
 		kwargs['changed'] = false
 	end