Validation.IsValid previously counted every problem regardless of severity,
leaving Severity inert. Now IsValid only fails on Error problems, so Warning
and Info no longer invalidate the result, and IsValidFor(Severity) lets
callers set the failing-severity threshold.
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.
