Add @Nullable where appropriate in JobExecution and StepExecution

Issue #4077

Author: acktsap

spring-batch-core/src/main/java/org/springframework/batch/core/JobExecution.java

@@ -38,6 +38,7 @@
  * @author Michael Minella
  * @author Mahmoud Ben Hassine
  * @author Dimitrios Liapis
+ * @author Taeik Lim
  *
  */
 @SuppressWarnings("serial")
@@ -139,6 +140,7 @@ public JobParameters getJobParameters() {
 	/**
 	 * @return The current end time.
 	 */
+	@Nullable
 	public Date getEndTime() {
 		return endTime;
 	}
@@ -164,6 +166,7 @@ public void setEndTime(Date endTime) {
 	/**
 	 * @return The current start time.
 	 */
+	@Nullable
 	public Date getStartTime() {
 		return startTime;
 	}
@@ -327,6 +330,7 @@ void addStepExecution(StepExecution stepExecution) {
 	 * @return a {@code Date} object representing the last time this
 	 *  {@code JobExecution} was updated.
 	 */
+	@Nullable
 	public Date getLastUpdated() {
 		return lastUpdated;
 	}

spring-batch-core/src/main/java/org/springframework/batch/core/StepExecution.java

@@ -273,6 +273,7 @@ public void setCreateTime(Date createTime) {
 	 *
 	 * @return the time when this execution started.
 	 */
+	@Nullable
 	public Date getStartTime() {
 		return startTime;
 	}
@@ -485,6 +486,7 @@ public void setProcessSkipCount(long processSkipCount) {
 	/**
 	 * @return the Date representing the last time this execution was persisted.
 	 */
+	@Nullable
 	public Date getLastUpdated() {
 		return lastUpdated;
 	}