async-hooks,net: ensure asyncId=null if no handle

If the .listen() hasn't been called on the server, there is no handle
object. In this case use null as the triggerAsyncId.

Fixes: nodejs#13548

lib/net.js

@@ -1558,7 +1558,8 @@ Server.prototype.getConnections = function(cb) {
   const self = this;
 
   function end(err, connections) {
-    nextTick(self[async_id_symbol], cb, err, connections);
+    const asyncId = self._handle ? self[async_id_symbol] : null;
+    nextTick(asyncId, cb, err, connections);
   }
 
   if (!this._usingSlaves) {

test/async-hooks/test-get-connections.js

@@ -0,0 +1,14 @@
+'use strict';
+
+const assert = require('assert');
+const net = require('net');
+const server = net.createServer();
+
+try {
+    server.getConnections(() => {
+        assert.ok(true, "No error thrown")
+    });
+}
+catch (e) {
+    assert.ok(false, "getConnections threw an error");
+}