Parse NULL geometry

[JosiahParry]

This PR updates parse_esri_json(). It now

• creates an empty geometry collection when a geometry is NULL
• when 0 features are returned an empty data.frame with the correct fields and types is returned.
  • Note that this does not propagate to arc_select() because there is an additional check in there.
• If an unknown geometry type is returned, an informative error is elicited via cli

Supported by new function ptype_tbl() which is a data.frame version of remote_ptype_tbl().

Closes https://github.com/R-ArcGIS/arcgislayers/issues/168