st/icon: Always show empty texture if both gicons are NULL

Commit 7ff7fb5d forgot to clear the
`priv->icon_texture` actor when returning from st_icon_update(), which
means we don't always switch to an empty icon if both gicon properties
are set to NULL.

Fix this and destroy the actor before returning early from
st_icon_update().

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1178


(cherry picked from commit 07deda59)