system-helper: Add default return at end of polkit rules

This is not a functional change: the default return value is equivalent
to polkit.Result.NOT_HANDLED. However, this makes the behaviour more
obvious.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #2354
Approved by: matthiasclasen
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f