error in the __call__ method of class Script

If ```client``` is a pipeline, and this pipeline is empty, python converts ```client``` to ```False``` when casting in bool because Class ```BasePipeline``` has a method ```__len__```. 
```client``` is reset to ```self.registered_client```, and the script is not executed in the pipeline as it should.
The correct test is to check if ```client``` equals ```None```.

see http://stackoverflow.com/questions/100732/why-is-if-not-someobj-better-than-if-someobj-none-in-python for bool conversion (first answer).