GH-2565 - Skip null properties on mapping/reading.

Allows to use default values in the entity classes.
Closes #2565

Author: meistermeier

src/main/java/org/springframework/data/neo4j/core/mapping/DefaultNeo4jEntityConverter.java

@@ -481,8 +481,10 @@ private PropertyHandler<Neo4jPersistentProperty> populateFrom(MapAccessor queryR
 				}
 			} else {
 				Object value = conversionService.readValue(extractValueOf(property, queryResult), typeInformation, property.getOptionalConverter());
-				Class<?> rawType = typeInformation.getType();
-				propertyAccessor.setProperty(property, getValueOrDefault(ownerIsKotlinType, rawType, value));
+				if (value != null) {
+					Class<?> rawType = typeInformation.getType();
+					propertyAccessor.setProperty(property, getValueOrDefault(ownerIsKotlinType, rawType, value));
+				}
 			}
 		};
 	}

src/test/java/org/springframework/data/neo4j/core/mapping/DefaultNeo4jEntityConverterTest.java

@@ -0,0 +1,75 @@
+package org.springframework.data.neo4j.core.mapping;
+
+import org.junit.jupiter.api.Test;
+import org.neo4j.driver.Value;
+import org.neo4j.driver.Values;
+import org.neo4j.driver.internal.InternalNode;
+import org.neo4j.driver.internal.types.InternalTypeSystem;
+import org.neo4j.driver.internal.value.NodeValue;
+import org.neo4j.driver.types.TypeSystem;
+import org.springframework.data.mapping.callback.EntityCallbacks;
+import org.springframework.data.mapping.model.EntityInstantiators;
+import org.springframework.data.neo4j.core.convert.Neo4jConversions;
+import org.springframework.data.neo4j.core.mapping.callback.EventSupport;
+import org.springframework.data.neo4j.core.schema.GeneratedValue;
+import org.springframework.data.neo4j.core.schema.Id;
+import org.springframework.data.neo4j.core.schema.Node;
+import org.springframework.data.util.ClassTypeInformation;
+
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.Map;
+
+import static org.assertj.core.api.AssertionsForClassTypes.assertThat;
+
+class DefaultNeo4jEntityConverterTest {
+
+	private final DefaultNeo4jEntityConverter entityConverter;
+
+	DefaultNeo4jEntityConverterTest() {
+		EntityInstantiators entityInstantiators = new EntityInstantiators();
+		NodeDescriptionStore nodeDescriptionStore = new NodeDescriptionStore();
+		DefaultNeo4jConversionService conversionService = new DefaultNeo4jConversionService(new Neo4jConversions());
+		Neo4jMappingContext context = new Neo4jMappingContext();
+		context.addPersistentEntity(ClassTypeInformation.from(EntityWithDefaultValues.class));
+		nodeDescriptionStore.put("User", (DefaultNeo4jPersistentEntity<?>) context.getNodeDescription(EntityWithDefaultValues.class));
+		EventSupport eventSupport = EventSupport.useExistingCallbacks(context, EntityCallbacks.create());
+		TypeSystem typeSystem = InternalTypeSystem.TYPE_SYSTEM;
+		this.entityConverter = new DefaultNeo4jEntityConverter(entityInstantiators, nodeDescriptionStore, conversionService, eventSupport, typeSystem);
+	}
+
+	@Test
+	void readEntityWithDefaultValuesWithEmptyPropertiesFromDatabase() {
+		Map<String, Value> properties = new HashMap<>();
+		NodeValue mapAccessor = new NodeValue(
+				new InternalNode(1L, Collections.singleton("EntityWithDefaultValues"), properties)
+		);
+
+		EntityWithDefaultValues readNode = entityConverter.read(EntityWithDefaultValues.class, mapAccessor);
+		assertThat(readNode).isNotNull();
+		assertThat(readNode.noDefaultValue).isNull();
+		assertThat(readNode.defaultValue).isEqualTo("Test");
+	}
+
+	@Test
+	void readEntityWithDefaultValuesWithPropertiesFromDatabase() {
+		Map<String, Value> properties = new HashMap<>();
+		properties.put("noDefaultValue", Values.value("valueFromDatabase1"));
+		properties.put("defaultValue", Values.value("valueFromDatabase2"));
+		NodeValue mapAccessor = new NodeValue(
+				new InternalNode(1L, Collections.singleton("EntityWithDefaultValues"), properties)
+		);
+
+		EntityWithDefaultValues readNode = entityConverter.read(EntityWithDefaultValues.class, mapAccessor);
+		assertThat(readNode).isNotNull();
+		assertThat(readNode.noDefaultValue).isEqualTo("valueFromDatabase1");
+		assertThat(readNode.defaultValue).isEqualTo("valueFromDatabase2");
+	}
+
+	@Node
+	static class EntityWithDefaultValues {
+		@Id @GeneratedValue Long id;
+		public String noDefaultValue;
+		public String defaultValue = "Test";
+	}
+}