cluster/serializer/broker: Fix handler lookup

Handler overwrites operator bool, so need to explicitly test for nullptr
rather than not having any handlers defined.

src/cluster/serializer/broker/Serializer.cc

@@ -76,7 +76,7 @@ std::optional<detail::Event> to_zeek_event(const broker::zeek::Event& ev) {
 
     zeek::Args vl;
     zeek::EventHandlerPtr handler = zeek::event_registry->Lookup(name);
-    if ( ! handler ) {
+    if ( handler == nullptr ) {
         zeek::reporter->Error("Failed to lookup handler for '%s'", std::string(name).c_str());
         return std::nullopt;
     }

testing/btest/Baseline/cluster.generic.publish-no-handler/..manager..stderr

@@ -0,0 +1,2 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+received termination signal

testing/btest/Baseline/cluster.generic.publish-no-handler/..manager.out

@@ -0,0 +1,3 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+node_up, worker-1
+node_down, worker-1

testing/btest/Baseline/cluster.generic.publish-no-handler/..worker..stderr

@@ -0,0 +1,2 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+received termination signal

testing/btest/Baseline/cluster.generic.publish-no-handler/..worker.out

@@ -0,0 +1,2 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+got finish

testing/btest/cluster/generic/publish-no-handler.zeek

@@ -0,0 +1,66 @@
+# @TEST-DOC: A worker receiving an event without a handler implemented would produce a reporter error
+#
+# @TEST-REQUIRES: have-zeromq
+#
+# @TEST-GROUP: cluster-zeromq
+#
+# @TEST-PORT: XPUB_PORT
+# @TEST-PORT: XSUB_PORT
+# @TEST-PORT: LOG_PULL_PORT
+#
+# @TEST-EXEC: cp $FILES/zeromq/cluster-layout-simple.zeek cluster-layout.zeek
+# @TEST-EXEC: cp $FILES/zeromq/test-bootstrap.zeek zeromq-test-bootstrap.zeek
+#
+# @TEST-EXEC: zeek --parse-only manager.zeek worker.zeek
+#
+# @TEST-EXEC: btest-bg-run manager "ZEEKPATH=$ZEEKPATH:.. && CLUSTER_NODE=manager zeek -b ../manager.zeek >out"
+# @TEST-EXEC: btest-bg-run worker "ZEEKPATH=$ZEEKPATH:.. && CLUSTER_NODE=worker-1 zeek -b ../worker.zeek >out"
+#
+# @TEST-EXEC: btest-bg-wait 30
+# @TEST-EXEC: btest-diff ./manager/out
+# @TEST-EXEC: btest-diff ./manager/.stderr
+# @TEST-EXEC: btest-diff ./worker/out
+# @TEST-EXEC: btest-diff ./worker/.stderr
+
+
+# @TEST-START-FILE common.zeek
+@load ./zeromq-test-bootstrap
+
+
+global hello: event() &is_used;
+global finish: event() &is_used;
+# @TEST-END-FILE
+
+# @TEST-START-FILE manager.zeek
+@load ./common.zeek
+event send_finish(id: string)
+	{
+	Cluster::publish(Cluster::nodeid_topic(id), finish);
+	}
+# If a node comes up that isn't us, send it a hello and
+# schedule sending a my_finish
+event Cluster::node_up(name: string, id: string)
+	{
+	print "node_up", name;
+	Cluster::publish(Cluster::nodeid_topic(id), hello);
+	schedule 20msec { send_finish(id) };
+	}
+
+# If the worker vanishes, finish the test.
+event Cluster::node_down(name: string, id: string)
+	{
+	print "node_down", name;
+	terminate();
+	}
+# @TEST-END-FILE
+
+# @TEST-START-FILE worker.zeek
+@load ./common.zeek
+# The worker does not implement hello!
+
+event finish()
+	{
+	print "got finish";
+	terminate();
+	}
+# @TEST-END-FILE