Avoid conversion when return value is null.

Closes #1167

Author: schauder

spring-data-jdbc/src/main/java/org/springframework/data/jdbc/repository/query/AbstractJdbcQuery.java

@@ -184,7 +184,7 @@ public Object mapRow(ResultSet rs, int rowNum) throws SQLException {
 
 			T object = delegate.mapRow(rs, rowNum);
 
-			return converter.convert(object);
+			return object == null ? null : converter.convert(object);
 		}
 	}
 }

spring-data-jdbc/src/test/java/org/springframework/data/jdbc/repository/JdbcRepositoryIntegrationTests.java

@@ -37,7 +37,6 @@
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
-
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.beans.factory.config.PropertiesFactoryBean;
 import org.springframework.context.ApplicationListener;
@@ -68,6 +67,7 @@
 import org.springframework.jdbc.core.JdbcTemplate;
 import org.springframework.jdbc.core.RowMapper;
 import org.springframework.jdbc.core.namedparam.NamedParameterJdbcTemplate;
+import org.springframework.lang.Nullable;
 import org.springframework.test.context.TestExecutionListeners;
 import org.springframework.test.context.junit.jupiter.SpringExtension;
 import org.springframework.test.jdbc.JdbcTestUtils;
@@ -549,6 +549,22 @@ void queryByAggregateReference() {
 		assertThat(result).extracting(e -> e.idProp).containsExactly(two.idProp);
 	}
 
+	@Test // GH-1167
+	void stringResult() {
+
+		repository.save(createDummyEntity()); // just ensure we have data in the table
+
+		assertThat(repository.returnInput("HELLO")).isEqualTo("HELLO");
+	}
+
+	@Test // GH-1167
+	void nullStringResult() {
+
+		repository.save(createDummyEntity()); // just ensure we have data in the table
+
+		assertThat(repository.returnInput(null)).isNull();
+	}
+
 	private Instant createDummyBeforeAndAfterNow() {
 
 		Instant now = Instant.now();
@@ -625,6 +641,10 @@ interface DummyEntityRepository extends CrudRepository<DummyEntity, Long> {
 
 		List<DummyEntity> findByRef(int ref);
 		List<DummyEntity> findByRef(AggregateReference<DummyEntity, Long> ref);
+
+		@Query("SELECT CAST(:hello AS CHAR(5)) FROM DUMMY_ENTITY")
+		@Nullable
+		String returnInput(@Nullable String hello);
 	}
 
 	@Configuration