Empty gmfs for scenarios starting from shakemaps and site_effects=sitemodel

[CatalinaYepes]

The engine is producing empty GMFS when running scenarios starting from ShakeMaps and site_effects=sitemodel.

In addition to fixing this bug, we should consider raising an error in OQ if a calculation has all nan in the GMFS before producing the outputs. Currently, the calculation runs without issues, but all outputs are zeros, and there are problems later on in the plugin due to the empty values.

Attached is a calculation to replicate this error. When setting site_effects=no or site_effects=shakemap, the corresponding GMFS are produced.

job.zip

[CatalinaYepes]

Shall we remove the parameter? See #4895