ToLookup Operator Doesn't Work With Null Keys (behaviour differs to IEnumerable ToLookup)

#### Bug

> Which library version?

Tested with both 3.0.0 (what we are currently using) and 5.0.0.

> What are the platform(s), environment(s) and related component version(s)?

Windows 10, .NET 4.8

> What is the use case or problem?

The ToLookup operator cannot be created or indexed with null keys at all, the IEnumerable LINQ version can be.

> What is the expected outcome?

Creation of an ILookup with a null key entry to match the behaviour of `var lookup = new string?[] { "foo", null }.ToLookup(x => x);`

Successful lookups using the indexer with a null key.

> What is the actual outcome?

An ArgumentNullException is thrown.

> What is the stacktrace of the exception(s) if any?

Trying to create with null key:

at System.Collections.Generic.Dictionary`2.FindValue(TKey key)
at System.Collections.Generic.Dictionary`2.TryGetValue(TKey key, TValue& value)
at System.Reactive.Lookup`2.Add(K key, E element) in /_/Rx.NET/Source/src/System.Reactive/Internal/Lookup.cs:line 30
at System.Reactive.Linq.ObservableImpl.ToLookup`3._.OnNext(TSource value) in /_/Rx.NET/Source/src/System.Reactive/Linq/Observable/ToLookup.cs:line 54

For indexing with null key:

at System.Collections.Generic.Dictionary`2.FindValue(TKey key)
at System.Collections.Generic.Dictionary`2.TryGetValue(TKey key, TValue& value)
at System.Reactive.Lookup`2.get_Item(K key) in /_/Rx.NET/Source/src/System.Reactive/Internal/Lookup.cs:line 40

> Do you have a code snippet or project that reproduces the problem?

On creation:

var lookup = await new string?[] { "foo", null }.ToObservable().ToLookup(x => x);

And on lookups:

var lookup = await new string?[] { "foo", "bar" }.ToObservable().ToLookup(x => x);
var value = lookup[null];

Both of these examples work if you use the IEnumerable ToLookup operator.