hello,

thank you for your continued work on this library.

i have an issue that for some reason only came up today, but i believe this is a similar issue to #1206. i have one test which only checks that a function was not called using AssertNotCalled(), which now fails because the mutex is nil and testify panics.

i instantiate the mock using new, is that wrong?

i am willing to prepare a PR for you, but i am not sure what the fix is. in #1207, a check is added:

	if m.mutex == nil {
		m.mutex = &sync.Mutex{}
	}

does this need to be added to all of AssertNotCalled(), AssertCalled(), AssertNumberOfCalls()? is that all or are there any other places?