client/systray: fix crash when suggested exit node has no location

The suggest-exit-node LocalAPI response omits the Location field when
the suggested exit node has no location, such as a regular tailnet exit
node. Building the exit node menu called CountryCode and City on the
invalid view unconditionally, crashing the app on startup. The crash
was introduced in aa21b0c00 (#19627), which added those calls.

Add an integration test that starts the systray app against a private
dbus session bus, a fake StatusNotifierWatcher, and a fake LocalAPI
serving such a suggestion, then verifies the StatusNotifierItem is
registered and the initial menu is built. On Linux the systray is pure
dbus (StatusNotifierItem and dbusmenu), so no X server or desktop
environment is needed and the test runs in existing CI.

Fixes #20678

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I262ae0f724cb7c41aa0f531fcbded967e5bee432