BUG: data handler fails if no chem variable is used

Bug

Error description

MLAir was started without any chemical as input (but still as target).

Error message

First guess on error origin

Error origin

Data handler cannot handle, if no chemical is included in input space when checking for negative concentrations. Problem is line 409 in data_handler_single_station:DataHandlerSingleStation.check_for_negative_concentrations

Solution

Add an if statement to check, if the used_chem_vars variable has a length of 1 or longer. Only apply clipping in this case.