Communication does this in waitForMessageSync if a filter
is not passed in.
This fixes a crash in icsneoWaitForRxMessagesWithTimeOut
for the legacy API.
For good measure, creation with an empty std::function will
immediately throw an std::bad_function_call back at the
caller, rather than letting that happen on the callback thread.
I'm also making the members const here so they are provably
always non-null (and not empty, for the function).
Paul Hollinsky
Kyle Schwarz