add minProperties / additionalProperties

Co-authored-by: Flo Edelmann <git@flo-edelmann.de>

Author: k-yle

schemas/field.json

@@ -375,7 +375,11 @@
                 },
                 "units": {
                     "type": "object",
-                    "description": "Defines the permitted units. The key is the ID used by CLDR. The value is the value used in the OSM tag. If there are multiple values, the first one will be preferred. Use an empty string if the unit is not included in the OSM tag."
+                    "description": "Defines the permitted units. The key is the ID used by CLDR. The value is the value used in the OSM tag. If there are multiple values, the first one will be preferred. Use an empty string if the unit is not included in the OSM tag.",
+                    "additionalProperties": {
+                      "type": "string"
+                    },
+                    "minProperties": 1
                 }
             },
             "allOf": [