Add some failsafes: don't let those special renderings render if they are not defined

assets/layers/drinking_water/drinking_water.json

@@ -131,7 +131,8 @@
       "render":{
         "en": "<a href='#{_closest_other_drinking_water_id}'>There is another drinking water fountain at {_closest_other_drinking_water_distance} meter</a>",
         "nl": "<a href='#{_closest_other_drinking_water_id}'>Er bevindt zich een ander drinkwaterpunt op {_closest_other_drinking_water_distance} meter</a>"
-      }
+      },
+      "condition": "_closest_other_drinking_water_id~*"
     }
   ]
 }