Fix criticals about unset GIO attributes (#1063)

The simplest solution seems to be to use g_file_info_get_attribute_*() APIs rather than specialized APIs such as g_file_info_get_is_symlink(), as they return the default value that specialized APIs returned before GLib 2.77.

Another possibility is to precede specialized API calls with g_file_info_has_attribute(), but this is more cumbersome. See for example https://gitlab.gnome.org/GNOME/gtk/-/commit/c1fa916e88de20fc61dc06d3ff9f26722effa0df

Fixes: #1063 (closed)