wpctl: accept 0 as ID in clear-default subcommand

Currently it is not possible to clear the default Audio/Sink node with:

$ wpctl clear-default 0

Because ID 0 is not accepted. This MR solves the problem by treating clear-default IDs differently than the rest of the wpctl subcommands.

Unlike the rest of subcommands in wpctl, The ID for the clear-default subcommand must be a setting ID, not a pipewire Object ID. This means we cannot use the parse_id() and translate_id() helper functions because the setting ID 0 is acceptable, and the @DEFAULT_AUDIO_SINK@, @DEFAULT_AUDIO_SOURCE@ and @DEFAULT_VIDEO_SOURCE@ strings are not acceptable.