Weaken assertion in PeerFinder

It can be inactive with no leader if it's handling an incoming PeersRequest
before being activated for the first time.

server/src/main/java/org/elasticsearch/discovery/PeerFinder.java

@@ -161,7 +161,7 @@ public abstract class PeerFinder {
                 peersRequest.getKnownPeers().stream().map(DiscoveryNode::getAddress).forEach(this::startProbe);
                 knownPeers = getFoundPeersUnderLock();
             } else {
-                assert leader.isPresent();
+                assert leader.isPresent() || lastAcceptedNodes == null;
                 knownPeers = Collections.emptyList();
             }
             return new PeersResponse(leader, knownPeers, currentTerm);