[#1550] Fix Identity insert query for Db2

Author: DavideD

hibernate-reactive-core/src/main/java/org/hibernate/reactive/id/insert/ReactiveAbstractReturningDelegate.java

@@ -9,6 +9,7 @@
 import java.util.concurrent.CompletionStage;
 
 import org.hibernate.dialect.CockroachDialect;
+import org.hibernate.dialect.DB2Dialect;
 import org.hibernate.dialect.Dialect;
 import org.hibernate.dialect.MySQLDialect;
 import org.hibernate.dialect.SQLServerDialect;
@@ -92,6 +93,11 @@ private static String createInsert(PreparedStatementDetails insertStatementDetai
 			}
 			return sql;
 		}
+		if ( dialect instanceof DB2Dialect ) {
+			// ORM query: select id from new table ( insert into IntegerTypeEntity values ( ))
+			// Correct  : select id from new table ( insert into LongTypeEntity (id) values (default))
+			return insertStatementDetails.getSqlString().replace( " values ( ))", " (" + identifierColumnName + ") values (default))" );
+		}
 		return insertStatementDetails.getSqlString();
 	}