Add overloads for Throw.ArgumentNull.WhenNull() for strings

[akamsteeg]

Currently, Throw.ArgumentNull.WhenNull() works on objects. However, a lot of parameters you'd want to check are strings. So, no you have to do Throw.ArgumentNull.WhenNull(string.IsNullOrEmpty(parameter), nameof(parameter)) which is silly. We should add overloads to Throw.ArgumentNull.WhenNull() that work on strings directly and do the null-or-empty check themselves.