Cleanup ! operator usage in FallbackIfEmpty

[sid-6581]

As part of reviewing nullable reference annotations (#803), this PR attempts to simplify usage of the ! operator in the internals of FallbackIfEmpty.cs. No public APIs are affected.

The following line from the code prior to this PR will cause a compiler warning CS8619 (nullability of reference types in source type doesn't match target type):

return FallbackIfEmptyImpl(source, 1, fallback, default!, default!, default!, null);

Because potential null values are passed to FallbackIfEmptyImpl, the return value will be IEnumerable<T?>.

One solution would be to use another ! at the end of the method call. However, the overuse of ! doesn't feel very clean. Also, the signature of FallbackIfEmptyImpl doesn't seem to signal intent as clearly as making the fallback values nullable. This PR also brings some consistency, since the fallback collection is already nullable.

Now that the fallback values are nullable, we can remove the ! from each of the defaults passed to FallbackIfEmptyImpl, but we need to use the ! operator on the fallback values when returning them. This seems a reasonable tradeoff to clean up this file, and should be safe considering the private accessibility of the method.

[atifaziz]

Also, the signature of FallbackIfEmptyImpl doesn't seem to signal intent as clearly as making the fallback values nullable. This PR also brings some consistency, since the fallback collection is already nullable.

Completely agree. This is much better!

Thanks!

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PS In #804, I introduced Defaultable<> that allows getting rid of ! completely but I think that's just being sneaky and I might revert.