tsrelay/handler: don't panic on nil CurrentTailnet or Self

[ZAG23]

getPeers checks st.CurrentTailnet for nil before copying it into the response, but then reads st.CurrentTailnet.MagicDNSSuffix unconditionally for every peer (get_peers.go:120). When tailscaled returns a status that lists peers but has no CurrentTailnet, each /peers poll panics and the connection is reset:

[tsrelay] http: panic serving 127.0.0.1:56614: runtime error: invalid memory address or nil pointer dereference
[tsrelay] github.com/tailscale-dev/vscode-tailscale/tsrelay/handler.(*handler).getPeers(...)
[tsrelay] 	/home/runner/work/vscode-tailscale/vscode-tailscale/tsrelay/handler/get_peers.go:120 +0x4d0
[error] could not poll for updates: d: request to http://127.0.0.1:56270/peers failed, reason: socket hang up

I hit this with v1.1.0 in VSCodium on macOS (standalone Tailscale app 1.102.4): 10 panics, one on its own and then nine consecutive 5-second polls about a minute and a half later, after which they stopped. I didn't determine why tailscaled briefly omitted CurrentTailnet.

Fix

• getPeers: when CurrentTailnet is nil, return the offline error if there is one (the logged-out case, unchanged), and otherwise return an error so the request fails like other status errors. Returning "CurrentTailnet": null alongside peers would just move the crash: the extension reads status.CurrentTailnet.Name without a null check in src/node-explorer-provider.ts. With a failed request, the poll throws before currentStatus is replaced, the same path a reset connection took before, so the Node Explorer keeps its last good tree and picks up changes on the next good poll.
• Defensive nil checks for Self in getPeers, getServe (which already checks Self a few lines earlier) and serveConfigDNS. tailscaled always sets Self, but -mockfile profiles don't have to.

Testing

• New tsrelay/handler/nil_status_test.go. Without the fix, four of its five tests panic, at get_peers.go:120, get_peers.go:140, get_serve.go:167 and create_serve.go:96; the logged-out test passes before and after. With the fix all five pass, along with go vet ./tsrelay/... and go test ./tsrelay/....
• Against a live tailnet, the full /peers and /serve JSON responses are identical to the v1.1.0 binary's.
• Run standalone with a -mockfile status that is running, lists peers, and has no CurrentTailnet: v1.1.0 panics, and this returns a plain-text 500 that the extension's resp.json() rejects, so the poll counts as failed.
• Built with Go 1.27 and GOEXPERIMENT=nojsonv2: 1.27 enables jsonv2 by default, and the pinned go-json-experiment/json doesn't compile against it. Not tested with ./tool/go.

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