inspector: fix inspector::Agent::HasConnectedSessions

PR-URL: https://github.com/nodejs/node/pull/20614
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/inspector_agent.cc

@@ -693,6 +693,8 @@ bool Agent::IsWaitingForConnect() {
 }
 
 bool Agent::HasConnectedSessions() {
+  if (client_ == nullptr)
+    return false;
   return client_->hasConnectedSessions();
 }