IconThemePath is not a standard property in XDG's StatusNotifierItem
specification, so missing this property should not be logged as an error.
This patch changes the log level to SWAY_DEBUG when swaybar queries the
value of IconThemePath so that swaybar won't log the returned message as
an error if IconThemePath does not exist.
Closes: https://github.com/swaywm/sway/issues/6092