ipn/store: [TestNewStore] do not use an empty file

Otherwise it would log warnings about an empty file.
```
    stores.go:138: store.NewFileStore("/tmp/3777352782"): file empty; treating it like a missing file [warning]
```

Signed-off-by: Maisem Ali <maisem@tailscale.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f