Stop area relations with only stops or platforms should be imported (#700)

Fixes #699

group: https://www.openstreetmap.org/relation/17749930 
stop area with only stops:
https://www.openstreetmap.org/relation/17749926

On http://localhost:8000/#view=17.27/41.643986/-0.89179
<img width="970" height="650" alt="image"
src="https://github.com/user-attachments/assets/487c0de0-a84d-412a-ac84-0aa6efb06516"
/>

Author: hiddewie

import/openrailwaymap.lua

@@ -1453,8 +1453,10 @@ function osm2pgsql.process_relation(object)
     for _, member in ipairs(object.members) do
       if member.role == 'stop' and member.type == 'n' then
         table.insert(stop_members, member.ref)
+        has_members = true
       elseif member.role == 'platform' then
         table.insert(platform_members, member.ref)
+        has_members = true
       elseif member.type == 'n' then
         -- Station has no role defined
         table.insert(node_members, member.ref)

import/test/test_import_stop_area.lua

@@ -54,6 +54,21 @@ assert.eq(osm2pgsql.get_and_clear_imported_data(), {
   },
 })
 
+osm2pgsql.process_relation({
+  tags = {
+    ['type'] = 'public_transport',
+    ['public_transport'] = 'stop_area',
+  },
+  members = {
+    { role = 'stop', type = 'n', ref = 1 },
+  },
+})
+assert.eq(osm2pgsql.get_and_clear_imported_data(), {
+  stop_areas = {
+    { stop_ref_ids = '{1}', way_ref_ids = '{}', platform_ref_ids = '{}', node_ref_ids = '{}' },
+  },
+})
+
 -- Stop area groups
 
 osm2pgsql.process_relation({