Processing: Guard registerExportFunctions

This ran with empty data which crashed

```
db  | 2024-11-18 16:24:38.343 GMT [2564197] ERROR:  syntax error at or near "," at character 439
db  | 2024-11-18 16:24:38.343 GMT [2564197] STATEMENT:  CREATE FUNCTION public."atlas_export_flatgeobuf_poiclassification"(region Geometry(Polygon))
db  | 	           RETURNS BYTEA
db  | 	           LANGUAGE plpgsql
db  | 	           AS $$
db  | 	           DECLARE
db  | 	            fgb BYTEA;
db  | 	           BEGIN
db  | 	            SELECT ST_AsFlatGeobuf(q, false, 'geom') INTO fgb FROM (
db  | 	              SELECT st_transform(geom, 4326) AS geom,
db  | 	                id,
db  | 	                osm_id,
db  | 	                osm_type::text,
db  | 	                ,
db  |
db  | 	              FROM "poiClassification"
db  | 	              WHERE geom && ST_Transform(region, 3857) AND minzoom > -1
db  | 	            ) q;
db  | 	            RETURN fgb;
db  | 	           END;
db  | 	           $$;
db  | 2024-11-19 09:15:43.994 GMT [2649964] ERROR:  syntax error at or near "," at character 439
```

app/src/registerSQLFunctions/registerExportFunctions.ts

@@ -23,6 +23,13 @@ export async function registerExportFunctions(tables: typeof exportApiIdentifier
       const tagKeys = tagKeyQuery.map(({ key }) => `tags->>'${key}' as "${key}"`).join(',')
       const metaKeys = metaKeyQuery.map(({ key }) => `meta->>'${key}' as "${key}"`).join(',')
 
+      if (!tagKeys || !metaKeys) {
+        console.error(
+          'Failed to `registerExportFunctions` because required tagKeys and metaKeys are empty',
+          { functionName, tagKeyQuery, metaKeyQuery },
+        )
+      }
+
       return geoDataClient.$transaction([
         geoDataClient.$executeRawUnsafe(`
           DROP FUNCTION IF EXISTS public."${functionName}"(region Geometry(Polygon));