Fix the debug message in Mall-PR

The function `map::place_vending` requires an item group as parameter (added in 48b13595120a10face47271c2dea3b5a041c0015). This item group is not loaded from json by the json mapgen system - `mapgen_function_json::setup_place_special` keeps `mapgen_function_json::type` empty. Therefor when the special is placed, it calls `map::place_vending` with an empty string as item group parameter, which leads to the debug message (as there is no such group).

I added a default value for the item group: "vending_food"/"vending_drink" (50/50 chance) - these item groups are already used by the hard coded spawns of this special.

It can be overridden with the "item_group" member in json:
```
{ "type": "vendingmachine", "x": 1, "y": 21, "item_group": "novels" }
```