fix(schematype): handle passing message function to SchemaType#validate() as positional arg

Fix #8360

Author: vkarpov15

lib/schematype.js

@@ -676,21 +676,24 @@ SchemaType.prototype.get = function(fn) {
 SchemaType.prototype.validate = function(obj, message, type) {
   if (typeof obj === 'function' || obj && utils.getFunctionName(obj.constructor) === 'RegExp') {
     let properties;
-    if (message instanceof Object && !type) {
+    if (typeof message === 'function') {
+      properties = { validator: obj, message: message };
+      properties.type = type || 'user defined';
+    } else if (message instanceof Object && !type) {
       properties = utils.clone(message);
       if (!properties.message) {
         properties.message = properties.msg;
       }
       properties.validator = obj;
       properties.type = properties.type || 'user defined';
     } else {
-      if (!message) {
+      if (message == null) {
         message = MongooseError.messages.general.default;
       }
       if (!type) {
         type = 'user defined';
       }
-      properties = {message: message, type: type, validator: obj};
+      properties = { message: message, type: type, validator: obj };
     }
 
     if (properties.isAsync) {