[BugFix] Only perform type evaluation if target value is not null or empty string

[homestar9]

See my comment on #45

[lmajano]

Would be great to have a note about this in the docs. Can you assist?

[homestar9]

Sure thing, I will be happy to help. At first, I thought it might be a good idea to include a note at the top of the https://coldbox-validation.ortusbooks.com/overview/valid-constraints page, but some constraints like required and udf don't ignore nulls or empty strings. Therefore, I believe we should add a note for each constraint that does ignore null/empty next to their description. It will take longer, but I think it will be more beneficial for end-users. I will add this item to my "todo" list.