if rule.message is null, it should be regarded as invalid, not only undefined (#20)

Co-authored-by: jilingy <jilingy@chanjet.com>

Author: jasonjiicloud

src/index.ts

@@ -201,7 +201,7 @@ class Schema {
           if (!options.suppressWarning && errorList.length) {
             Schema.warning('async-validator:', errorList);
           }
-          if (errorList.length && rule.message !== undefined) {
+          if (errorList.length && rule.message !== undefined && rule.message !== null) {
             errorList = [].concat(rule.message);
           }