Add explicit check against null and undefined

Author: bpiggin

src/classes/Entity.ts

@@ -758,7 +758,7 @@ class Entity<
 
     // Check for required fields
     Object.keys(required).forEach(field =>
-      required[field] && !data[field]
+      required[field] && (data[field] === undefined || data[field] === null)
         && error(`'${field}${this.schema.attributes[field].alias ? `/${this.schema.attributes[field].alias}` : ''}' is a required field`)
     ) // end required field check