Typings for find don't include undefined #3969
Description
Bug Report
Current Behavior
The overload signatures for find don't reflect the operator's behaviour.
The return types are OperatorFunction<T, S> and MonoTypeOperatorFunction<T>.
However, the operator will emit undefined if no match is found. See this test.
Possible Solution
The return types should be OperatorFunction<T, S | undefined> and OperatorFunction<T, T | undefined>.