Room schema: Remove "quantity" default value and add minimum

The default value was a string, so remote it. Provide a reasonable
minimum value.

Author: ampleyfly

schema/m3-room.schema.json

@@ -2100,7 +2100,7 @@
           "quantity": {
             "type": "integer",
             "title": "Quantity",
-            "default": "",
+            "minimum": 1,
             "examples": [3]
           },
           "homeNodes": {