Give a better error message than RuntimeError: empty maps are not supported by the schema.

[Michele Simionato]

This happens since the region specified in the risk job might not be covering any assets. It is triggered by a command like

 $ openquake --export-risk 245 Th_risk

(reported here: https://groups.google.com/forum/?hl=en#!topic/openquake-users/Ar0uU-Q22iM)

[Michele Simionato (michele-simionato)]

There is a validation for that case in ./openquake/engine/calculators/risk/validation.py. It should be moved to commonlib, but that is another ticket.
Here we could put a check on the exporter. It is best not to do anything until this happens again and we have access to the input files, so that we can see what's the real problem.