gtkplacessidebar.c: add bookmark null checking
This fixes nautilus crash and perhaps other callers issues. Nautilus (and sometimes glib) crashes with malformed URI inside of the bookmarks file .config/gtk-3.0/bookmarks when it has no LABEL.
This is a result from the MR glib!2065 (closed) analysis and agreement. glib!2065 (comment 1091979)
Signed-off-by: Geyslan G. Bem geyslan@gmail.com