nullref in SpecializedNamespace.get_Overloads

[jakebailey]

Seeing this one pop up. It looks like the checks on _original being null are not consistent in this file.

Also, the statements ending in ?.MaybeEnumerate() are likely not needed either. ripgrepping shows:

$ rg '\?\.MaybeEnumerate'
src\Analysis\Engine\Impl\ModuleAnalysis.cs
356:            if (v.Variable?.Types?.MaybeEnumerate().FirstOrDefault() is ParameterInfo) {

src\Analysis\Engine\Impl\Values\SpecializedNamespace.cs
212:        public override IEnumerable<ILocationInfo> Locations => _original?.Locations?.MaybeEnumerate();
220:        internal override IEnumerable<ILocationInfo> References => _original.References?.MaybeEnumerate();

I have a fix ready, just making this for tracking.