fix(@embark/pipeline): check if config arg of writeStats is falsy

When passing the name of a build pipeline that doesn't exist, e.g.
`embark run|build --pipeline foo`, `writeStats` would cause an unhandled
promise rejection because its config argument is falsy. Checking for that and
returning early from `writeStats` allows the `"no webpack config has the name
'foo'"` error to be reported correctly.

packages/embark-pipeline/src/webpackProcess.js

@@ -61,7 +61,7 @@ class WebpackProcess extends ProcessWrapper {
   }
 
   async writeStats(config, stats, callback){
-    if (!config.stats || config.stats === 'none') {
+    if (!config || !config.stats || config.stats === 'none') {
       return callback();
     }
     try {