As part of the overall async chain stabilisation, I tend to prefer _explosion on missing next()_ over code that could hide an actually incompletely plumbed async chain.

Said more clearly, I would like to remove all of the:

```javascript
if (next) {
   next();
}
```

...and replacing these callee's securities by caller's injection of dummy `function() {/* nothing to do*/}` as you do at the beginning of this PR (consolidated view).