gfileinfo: Add missing (nullable) annotation to get_attribute_as_string()
gfileinfo: Add missing (nullable) annotation to get_attribute_as_string()

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

Fixes: #1790

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib