Fix null errors when evaluating style expressions (#186)

Problems solved:
- nulls when evaluating station count
- nulls when evaluating maxspeed hover for highspeed lines


![image](https://github.com/user-attachments/assets/21c58821-ccba-4aaa-82e2-65f4b2caebbd)


![image](https://github.com/user-attachments/assets/340f8ff0-9c4b-4c40-aa2d-4bb77d571ae4)

features/stations.yaml

@@ -12,6 +12,7 @@ features:
     feature: tram_stop
     example:
       name: Llacuna
+      count: 2
 
   - description: Service station
     feature: service_station

import/sql/tile_views.sql

@@ -338,6 +338,26 @@ CREATE OR REPLACE FUNCTION signals_signal_boxes(z integer, x integer, y integer)
     WHERE way IS NOT NULL
   );
 
+-- Function metadata
+DO $do$ BEGIN
+  EXECUTE 'COMMENT ON FUNCTION signals_signal_boxes IS $tj$' || $$
+  {
+    "vector_layers": [
+      {
+        "id": "signals_signal_boxes",
+        "fields": {
+          "id": "integer",
+          "osm_id": "integer",
+          "feature": "string",
+          "ref": "string",
+          "name": "string"
+        }
+      }
+    ]
+  }
+  $$::json || '$tj$';
+END $do$;
+
 CREATE OR REPLACE VIEW signals_railway_signals AS
   SELECT
     id,

proxy/js/styles.mjs

@@ -768,7 +768,7 @@ const endHue = 284;
 
 const speedColor = theme => ['case',
   ['boolean', ['feature-state', 'hover'], false], ['case',
-    ['all', ['>=', ['get', 'maxspeed'], 260], ['<=', ['get', 'maxspeed'], 300]], colors[theme].hover.alternative,
+    ['all', ['!=', ['get', 'maxspeed'], null],  ['>=', ['get', 'maxspeed'], 260], ['<=', ['get', 'maxspeed'], 300]], colors[theme].hover.alternative,
     colors[theme].hover.main,
   ],
   ['==', ['get', 'maxspeed'], null], 'gray',
@@ -2221,7 +2221,7 @@ const layers = Object.fromEntries(knownThemes.map(theme => [theme, {
           ['get', 'name'],
           15,
           ['case',
-            ['>', ['get', 'count'], 1], ['concat', ['get', 'name'], ' (', ['get', 'count'], ')'],
+            ['>', ['coalesce', ['get', 'count'], 0], 1], ['concat', ['get', 'name'], ' (', ['get', 'count'], ')'],
             ['get', 'name'],
           ],
         ],
@@ -2254,7 +2254,7 @@ const layers = Object.fromEntries(knownThemes.map(theme => [theme, {
           ['get', 'name'],
           15,
           ['case',
-            ['>', ['get', 'count'], 1], ['concat', ['get', 'name'], ' (', ['get', 'count'], ')'],
+            ['>', ['coalesce', ['get', 'count'], 0], 1], ['concat', ['get', 'name'], ' (', ['get', 'count'], ')'],
             ['get', 'name'],
           ],
         ],