ipn/ipnlocal: fix a nil pointer dereference when serving /localapi/v0/tka/status

Fixes #13330

Signed-off-by: Nick Khyl <nickk@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f