GestureConverter: Do not use NotifyArgs as an optional return type

NotifyArgs is an std::variant, which is not an optional type.
Default-constructing a value of that type results in the creation of the
first type in the variant, which in this case is a
NotifyInputDevicesChangedArgs. It is incorrect to send that event event
when we intend to send no event.

Bug: 281671810
Test: atest inputflinger_tests
Change-Id: Ib78b3c354129f8494ee52115b2b99e210ceba6bf