Check if the execution of the samplingGeometry extent query should be executed (improve performance):
- Add pre-query to check if the observation table contains not null samplingGeometries.
- If only samplingGeometries with null value are contained, the extent query is not executed during the offering cache update.
Merge pull request #128 from CarstenHollmann/fix/check_if_samplingGeometry_not_null

Check if the execution of the samplingGeometry extent query should be executed