Don't use instance of for simple null checks

Fix eclipse-platform/eclipse.platform.releng.aggregator#2739

Author: laeubi

runtime/bundles/org.eclipse.core.jobs/src/org/eclipse/core/internal/jobs/JobManager.java

@@ -722,8 +722,9 @@ private void doShutdown() {
 		if (toCancel != null) {
 			for (Job job : toCancel) {
 				String jobName = printJobName(job) + " " + printState(job); //$NON-NLS-1$
-				if (job.getThread() instanceof Thread t) {
-					StackTraceElement[] stackTrace = t.getStackTrace();
+				Thread thread = job.getThread();
+				if (thread != null) {
+					StackTraceElement[] stackTrace = thread.getStackTrace();
 					for (StackTraceElement stackTraceElement : stackTrace) {
 						jobName += "\n\t at " + stackTraceElement; //$NON-NLS-1$
 					}