luci-base: form.js: fix TableSection descriptions row rendering

In some cases, a table cell at actions column of a descriptions row
may not be rendered. For example, this happens for GridSection when
sorting is disabled:

    s = m.section(form.GridSection, 'section_type');
    s.sortable = false;

Signed-off-by: Anton Kikin <a.kikin@tano-systems.com>