NIFI-6711 Clear secret key when auto restarting in order to obtain new secret key from the NiFi process

This closes #3780.

Signed-off-by: Kevin Doran <kdoran@apache.org>

nifi-bootstrap/src/main/java/org/apache/nifi/bootstrap/RunNiFi.java

@@ -1200,6 +1200,7 @@ public class RunNiFi {
                     }
 
                     defaultLogger.warn("Apache NiFi appears to have died. Restarting...");
+                    secretKey = null;
                     process = builder.start();
                     handleLogging(process);