Fix test-record metadata (resource_type)

InvenioRDM (November Release) runs into errors when the resource_type
does not expect a subtype, but an empty subtype is specified

upload/test-record.json

 {
   "metadata": {
-    "resource_type": { "type": "other", "subtype": "" },
+    "resource_type": { "type": "other" },
     "title": "Test Record without Files",
     "description": "A test record following the metadata schema for InvenioRDM November 2020 release (https://inveniordm.docs.cern.ch/reference/metadata/)",
     "creators": [

upload/test-record/metadata.json

 {
   "metadata": {
-    "resource_type": { "type": "other", "subtype": "" },
+    "resource_type": { "type": "other" },
     "title": "Test Record with Files",
     "description": "A test record following the metadata schema for InvenioRDM November 2020 release (https://inveniordm.docs.cern.ch/reference/metadata/)",
     "creators": [