feat(hasMany): addAssociation is no-op if passed false/null/undefined

lib/associations/has-many.js

@@ -367,6 +367,9 @@ module.exports = (function() {
     };
 
     obj[this.accessors.add] = function(newInstance, additionalAttributes) {
+      // If newInstance is null or undefined, no-op
+      if (!newInstance) return Utils.Promise.resolve();
+
       var instance = this
         , primaryKeyAttribute = association.target.primaryKeyAttribute;