Improve stability of zensus data in testmode

[ClaraBuettner]

When the zensus data is not cut correctly for the testmode (e.g. because the workflow was stopped by the user) it can happen that the created input files are created, but only have a header without any data.
This results in problems in every run because the files are only created once.
To avoid this, I will implement two things:

• check the size of the reduced input file from zensus; re-create it if size <1MB
• wrap a function around selects from database which throws a warning if no data was selected