Be mindful of non-existent actors

We have observed the following splat in SFT:

java.lang.NullPointerException: null
        at org.opendaylight.controller.eos.akka.registry.listener.type.EntityTypeListenerRegistry.onUnregisterListener(EntityTypeListenerRegistry.java:69) ~[bundleFile:?]
        at akka.actor.typed.javadsl.BuiltReceive.receive(ReceiveBuilder.scala:213) ~[bundleFile:?]

This this by checking whether we actually removed a listener.

Change-Id: Ifc80ae650db05fd1031af074c694588034f6903a
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>