luci-app-statistics: stat-genconfig: don't emit bools for absent uci options

Do not emit native collectd.conf boolean false options if the corresponding
uci option is unset in order to honour collectd's implicit defaults.

This fixes certain options, such as the memory plugins `ValuesAbsolute`,
being without effect if declared as default-enabled in the related plugin
uci forms.

Fixes: #5777
Signed-off-by: Jo-Philipp Wich <jo@mein.io>