NIFI-11963 Added null check in system test for instance cache method

- Timing issues in system integration tests could sometimes fail with a NullPointerException when calling the poison method multiple times

This closes #7618

Signed-off-by: David Handermann <exceptionfactory@apache.org>

nifi-system-tests/nifi-system-test-suite/src/test/java/org/apache/nifi/tests/system/NiFiInstanceCache.java

@@ -103,6 +103,9 @@ public class NiFiInstanceCache {
         }
 
         public void poison(final NiFiInstance toPoison) {
+            if (this.instance == null) {
+                return;
+            }
             final NiFiInstance rawInstance = this.instance.getRawInstance();
 
             if (Objects.equals(rawInstance, toPoison)) {