[reggen] Improve rendered documentation when no interrupts/alerts
[reggen] Improve rendered documentation when no interrupts/alerts

It seems that we always end up with an alert_list and interrupt_list,
but it might be empty.

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan