fix: config does not have to be defined

This is a fix for bug introduced in 9bdcae2

Author: vojtajina

src/adapter.js

@@ -80,7 +80,7 @@ var createMochaReporterConstructor = function(tc) {
 
 var createMochaStartFn = function(mocha) {
   return function(config) {
-    if(config.grep){
+    if(config && config.grep){
       mocha.grep(config.grep);
     }
     mocha.run();