luci-base: form.js: fix anonymous section table titles

When no section title is defined, the table renderer is supposed to use
the uci section id as fallback when the table section is not declared
to be anonymous.

Fixes: #3147
Fixes: a90bf384

 ("luci-base: form.js: harmonize title property handling")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>