Fixes bugs where the TypeExtractor throws an NPE instead of the operators

If the user accidentally passes ```null``` instead of e.g. a MapFunction. The TypeExtractor throws the NPE instead of the higher layer.

---------------- Imported from GitHub ----------------
Url: https://github.com/stratosphere/stratosphere/pull/896
Created by: twalthr
Labels:
Created at: Mon Jun 02 14:45:14 CEST 2014
State: closed