[datapackage_tabular-data] Bug: When patching a map via api/v2/ it always receives `"subtype": "tabular-collection"`

[mwallschlaeger]

Expected Behavior

After patching an existing map on a geonode branch: datapackage_tabular-data the maps subtype changes to "tabular-collection" if the patch does not contain any maplayers, instead it should keep the previous subtype.

[ridoo]

@mwallschlaeger I tend to move this issue to https://github.com/GeoNodeUserGroup-DE/geonode-dev-datapackage and close this issue section (until we really need it) to avoid upstream issues piling up here. What do you think?