Union doesn't dispose enumerator for empty lists

[Bouke]

UnionAsyncIterator has the same issue as reported for UnionIterator and fixed in .NET 5. It seems as simple as moving the call to SetEnumeratorAsync up just before the if.

reactive/Ix.NET/Source/System.Linq.Async/System/Linq/Operators/Union.cs

Lines 127 to 134 in bc34728

	++_index;
	var enumerator = enumerable.GetAsyncEnumerator(_cancellationToken);
	if (await enumerator.MoveNextAsync().ConfigureAwait(false))
	{
	await SetEnumeratorAsync(enumerator).ConfigureAwait(false);
	StoreFirst();

vs

        IEnumerator<TSource> enumerator = enumerable.GetEnumerator();
        SetEnumerator(enumerator);
        
        ++_state;
        if (enumerator.MoveNext())
        {
            StoreFirst();

(source: https://github.com/dotnet/runtime/blob/714a4420805ed53c311b05381c83c88894100fa9/src/libraries/System.Linq/src/System/Linq/Union.cs#L172-L178)

[idg10]

Thanks. I added a few tests to confirm the problem, and then verified that the fix you suggest does seem to fix it.

There are a fair few items on the backlog for Async LINQ. I'm hoping to get through a few more of them before doing another release, but if this is blocking you, let me know, and I can do a release without waiting to get any other fixes in.