Determine if "impossible" case in ReflectionExtensionMethods are testable

[kevin-montrose]

Cesil has has extension methods in ReflectionExtensionMethods for use in reflection-y code that DRY things up and makes sure nulls are caught early.

There are four places where a null check is necessary, according to nullable annotations in the BCL, that are currently untested. If these can be tested, they should be - if not, the todos should be replaced with comments explaining why they are impossible and null forgiveness operators (ie. !) used instead of the ifs.