Fix warning when using nullable reference types with OfType operator

[pellet]

Bug

Despite our best efforts, bugs can slip into releases or corner cases forgotten about. We will try our best to remedy the situation
and/or provide workarounds. Note that certain (odd) behaviors are by design and as such are not considered bugs.

Which library version?
v5.0

What are the platform(s), environment(s) and related component version(s)?
.net5.0

What is the use case or problem?
I want to use the OfType operator for filtering out nulls like so:

Observable
    .Return<string?>("blah")
    .OfType<string>()
    .SelectMany(this.audioService.PlayAudioPair)
    .Subscribe();

What is the expected outcome?
No warnings

What is the actual outcome?
When compiled has a warning:
warning CS8620: Argument of type 'IObservable<string?>' cannot be used for parameter 'source' of type 'IObservable<object>' in 'IObservable<string> Observable.OfType<string>(IObservable<object> source)' due to differences in the nullability of reference types.

[pellet]

I've found the operator .WhereNotNull() is working for me without warnings, but it is from ReactiveUI, it would be nice to have the OfType() operator handling nulls too or something similar in RX if possible.