only calling onClose if it's not nil

[WendelHime]

fix getlantern/engineering#2551

[Copilot]

Pull Request Overview

This PR adds nil checks before calling the onClose callback function in two methods to prevent potential nil pointer dereferences.

• Added nil checks for the onClose parameter in NewConnectionEx and NewPacketConnectionEx methods
• Ensures safe execution of the callback function when it might be nil

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