Update layer.js

Check if it's null or undefifned

Author: josete89

lib/layer.js

@@ -25,7 +25,9 @@ function zipRequirements() {
  * @return {Promise} empty promise
  */
 function createLayers() {
-  this.serverless.service.layers['pythonRequirements'] = {}
+  if(!this.serverless.service.layers['pythonRequirements']){
+     this.serverless.service.layers['pythonRequirements'] = {}
+  }
   this.serverless.service.layers['pythonRequirements'] = Object.assign(
     {
       artifact: path.join('.serverless', 'pythonRequirements.zip'),