diagnostics_channel: handle last subscriber removal

When iterating over diagnostics channel subscribers, assume their count
is zero if the list of subscribers becomes undefined, because there may
be only one subscriber which may unsubscribe itself as part of its
onMessage handler.

lib/diagnostics_channel.js

@@ -136,7 +136,7 @@ class ActiveChannel {
   }
 
   publish(data) {
-    for (let i = 0; i < this._subscribers.length; i++) {
+    for (let i = 0; i < (this._subscribers?.length || 0); i++) {
       try {
         const onMessage = this._subscribers[i];
         onMessage(data, this.name);

test/parallel/test-diagnostics-channel-sync-unsubscribe.js

@@ -0,0 +1,14 @@
+'use strict';
+
+const common = require('../common');
+const dc = require('node:diagnostics_channel');
+
+const channel_name = 'test:channel';
+const published_data = 'some message';
+
+const onMessageHandler = common.mustCall(() => dc.unsubscribe(channel_name, onMessageHandler));
+
+dc.subscribe(channel_name, onMessageHandler);
+
+// This must not throw.
+dc.channel(channel_name).publish(published_data);