Updated the DB2 Script to fix the not null in primary key.

resolves #500

spring-cloud-task-core/src/main/resources/org/springframework/cloud/task/schema-db2.sql

@@ -29,8 +29,8 @@ CREATE TABLE TASK_TASK_BATCH (
 CREATE SEQUENCE TASK_SEQ AS BIGINT START WITH 0 MINVALUE 0 MAXVALUE 9223372036854775807 NOCACHE NOCYCLE;
 
 CREATE TABLE TASK_LOCK  (
-	LOCK_KEY CHAR(36),
-	REGION VARCHAR(100),
+	LOCK_KEY CHAR(36) NOT NULL,
+	REGION VARCHAR(100) NOT NULL,
 	CLIENT_ID CHAR(36),
 	CREATED_DATE TIMESTAMP NOT NULL,
 	constraint LOCK_PK primary key (LOCK_KEY, REGION)