[JENKINS-73011] Round-trip JNLPLauncher.tunnel to null not ""

[jglick]

See JENKINS-73011. Amends #8762 + #8793.

Testing done

Automated test; also confirmed bug interactively in 2.426.3 → 2.440.2.

Proposed changelog entries

• After reconfiguring a static inbound agent in the GUI using fields such as WebSocket deprecated in 2.440.x, the suggested launch instructions would incorrectly include -tunnel (with no argument) even if that field had been left blank.

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[MarkEWaite]

This PR is now ready for merge. We will merge it after approximately 24 hours if there is no negative feedback.

/label ready-for-merge