Add Non-Empty Validator for Data Model Strings

[coreybutler]

From ngn-core created by coreybutler : ngnjs/ngn-core#7

Add an optional non-empty validator for strings that may be blank. Without this, it's possible to have a zero-length string, which will pass String type validation and meet the requirements of not being null. However; there are enough use cases where a non-empty string are required that it should be part of the core library.