Rule<T,TProperty>.Validate cast a null instance to T and validated it, so a
reference T whose accessor dereferences the instance threw
NullReferenceException on a direct validator.Validate(context) with a null
instance. Guard the null-instance path and return no problems instead.
Passing a null bound to GreaterThan/LessThan/Between etc. called
value.CompareTo(null), throwing NullReferenceException for non-null
reference-type values. Short-circuit when the bound is null so the rule is
satisfied instead of crashing.
Result/Result<T> equality treats any two failures as equal regardless of their
Error, which was undocumented and surprising for failure-specific branching.
The doc comment referenced `Geekeey.Extensions.Result.Prelude` but the actual
namespace is `Geekeey.Request.Result`, so the suggested using directive would
not compile.
