ERR_INVALID_ARG_TYPE when specifying `-o` flag

[louwers]

geostyler-cli -t mapbox  Wegdeel_Vlak_style.sld    

results in the (invalid) output geostyler/geostyler-mapbox-parser#334

 geostyler-cli -t mapbox -o style.json Wegdeel_Vlak_style.sld

Results in the following error:

✖ Error during translation of file "Wegdeel_Vlak_style.sld": TypeError [ERR_INVALID_ARG_TYPE]: The "data" argument must be of type string or an instance of Buffer, TypedArray, or DataView. Received an instance of Object

[KaiVolland]

Maybe specifing the source with -s sld helps. But we will have a look at this.