fix(max-pipe): Added null check

Author: flauc

package.json

@@ -1,6 +1,6 @@
 {
   "name": "angular2-notifications",
-  "version": "0.4.48",
+  "version": "0.4.49",
   "description": "An easy to use notification library for angular 2",
   "repository": {
     "type": "git",

src/max.pipe.ts

@@ -3,6 +3,8 @@ import {Pipe, PipeTransform} from '@angular/core';
 @Pipe({name: 'max'})
 export class MaxPipe implements PipeTransform {
   transform(value: string, ...args: any[]): any {
+    if (!value) return value;
+
     let allowed = args[0];
     let received = value.length;