Fix handling of string typed null values (#987)

* Added unit test for handling of missing (e.g. null) values

- When missing item is NOT typed, than it is properly handld as null,
  e.g. it is NOT included in the generated feature
- When missing item is typed to 'string', it gets converted into "null"
  string value hence generated featire has attribute
  non_existent_typed=null

* Fixed handling of missing or null values for attributes with type 'string'

* clean-up: removed unused import

planetiler-core/src/main/java/com/onthegomap/planetiler/expression/DataType.java

@@ -2,15 +2,14 @@
 
 import com.onthegomap.planetiler.reader.WithTags;
 import com.onthegomap.planetiler.util.Parse;
-import java.util.Objects;
 import java.util.function.BiFunction;
 import java.util.function.UnaryOperator;
 
 /**
  * Destination data types for an attribute that link the type to functions that can parse the value from an input object
  */
 public enum DataType implements BiFunction<WithTags, String, Object> {
-  GET_STRING("string", WithTags::getString, Objects::toString),
+  GET_STRING("string", WithTags::getString, Parse::parseStringOrNull),
   GET_BOOLEAN("boolean", WithTags::getBoolean, Parse::bool),
   GET_DIRECTION("direction", WithTags::getDirection, Parse::direction),
   GET_LONG("long", WithTags::getLong, Parse::parseLongOrNull),

planetiler-core/src/main/java/com/onthegomap/planetiler/util/Parse.java

@@ -31,6 +31,11 @@ public class Parse {
 
   private Parse() {}
 
+  /** Returns {@code tag} as a string or null if null. */
+  public static String parseStringOrNull(Object tag) {
+    return tag == null ? null : tag.toString();
+  }
+
   /** Returns {@code tag} as a long or null if invalid. */
   public static Long parseLongOrNull(Object tag) {
     return tag == null ? null : tag instanceof Number number ? Long.valueOf(number.longValue()) :

planetiler-custommap/src/test/java/com/onthegomap/planetiler/custommap/ConfiguredFeatureTest.java

@@ -267,6 +267,15 @@ void testTagValueAttributeTest() {
     }, 1);
   }
 
+  @Test
+  void testTagNullValueAttributeTest() {
+    testPolygon(TEST_RESOURCE, "tag_attribute_null.yml", waterTags, f -> {
+      var attr = f.getAttrsAtZoom(14);
+      assertNull(attr.get("non_existent"));
+      assertNull(attr.get("non_existent_typed"));
+    }, 1);
+  }
+
   @Test
   void testTagIncludeAttributeTest() {
     testPolygon(TEST_RESOURCE, "tag_include.yml", waterTags, f -> {

planetiler-custommap/src/test/resources/validSchema/tag_attribute_null.yml

@@ -0,0 +1,19 @@
+schema_name: Test Case Schema
+schema_description: Test case tile schema
+attribution: Test attribution
+sources:
+  osm:
+    type: osm
+    url: geofabrik:rhode-island
+layers:
+- id: testLayer
+  features:
+  - source:
+    - osm
+    geometry: polygon
+    include_when:
+      natural: water
+    attributes:
+    - key: non_existent
+    - key: non_existent_typed
+      type: string