Prevent null values from getting into the parameter lists.

According to https://neo4j.com/docs/cypher-manual/current/syntax/working-with-null/#cypher-null-intro
the result will always be empty since null != null.
Warn the user that the parameter value is not suitable for the query
but don't suppress its execution.
It might get called via user's input values or similar.

spring-data-neo4j/src/main/java/org/neo4j/springframework/data/repository/query/Neo4jQuerySupport.java

@@ -26,11 +26,14 @@ import java.util.List;
 import java.util.Map;
 import java.util.function.BiFunction;
 
+import org.apache.commons.logging.LogFactory;
 import org.neo4j.driver.Record;
+import org.neo4j.driver.Values;
 import org.neo4j.driver.types.TypeSystem;
 import org.neo4j.springframework.data.core.convert.Neo4jSimpleTypes;
 import org.neo4j.springframework.data.core.mapping.Neo4jMappingContext;
 import org.neo4j.springframework.data.repository.query.Neo4jQueryMethod.Neo4jParameters;
+import org.springframework.core.log.LogAccessor;
 import org.springframework.data.domain.Range;
 import org.springframework.data.geo.Circle;
 import org.springframework.data.geo.Distance;
@@ -54,6 +57,8 @@ abstract class Neo4jQuerySupport {
 	protected final Neo4jQueryMethod queryMethod;
 	protected final Class<?> domainType;
 
+	private static final LogAccessor log = new LogAccessor(LogFactory.getLog(Neo4jQuerySupport.class));
+
 	Neo4jQuerySupport(Neo4jMappingContext mappingContext, Neo4jQueryMethod queryMethod) {
 
 		Assert.notNull(mappingContext, "The mapping context is required.");
@@ -111,6 +116,16 @@ abstract class Neo4jQuerySupport {
 	 */
 	final Object convertParameter(Object parameter) {
 
+		if (parameter == null) {
+			// According to https://neo4j.com/docs/cypher-manual/current/syntax/working-with-null/#cypher-null-intro
+			// it does not make any sense to continue if a `null` value gets into a comparison
+			// but we just warn the users and do not throw an exception on `null`.
+			log.warn("Do not use `null` as a property value for comparison."
+				+ " It will always be false and return an empty result.");
+
+			return Values.NULL;
+		}
+
 		// Maybe move all of those into Neo4jConverter at some point.
 		if (parameter instanceof Range) {
 			return convertRange((Range) parameter);

spring-data-neo4j/src/test/java/org/neo4j/springframework/data/integration/imperative/RepositoryIT.java

@@ -1198,6 +1198,12 @@ class RepositoryIT {
 			.contains(person1);
 	}
 
+	@Test
+	void findBySimplePropertyByEqualsWithNullShouldWork() {
+		int emptyResultSize = 0;
+		assertThat(repository.findAllBySameValue(null)).hasSize(emptyResultSize);
+	}
+
 	@Test
 	void findBySimplePropertiesAnded() {