Check that files exist before tying to open them in GDE_check_consistency.py and GDE_check_OQ_input_files.py
OpenQuake input files for left-over buildings in cells are not created if there are no left-over buildings. However, the code attempts to open these files anyway (e.g. https://git.gfz-potsdam.de/dynamicexposure/legacy/gde_calculations_prototype/-/blob/master/GDE_check_consistency.py#L367, https://git.gfz-potsdam.de/dynamicexposure/legacy/gde_calculations_prototype/-/blob/master/GDE_check_OQ_input_files.py#L64). The code should first check if the file exists.