docs/templates/_footer: fix condition for emitting <br>
docs/templates/_footer: fix condition for emitting <br>

The footer has unnecessary empty vertical space.

This is because if the `last-update-label` attribute is specified, an
extra <br> is added, which is meant to separate the "Last updated" line
from the document revision number line.

However, if the document has no revision number, then it would seem that
the footer has unnecessary empty vertical space.

Fix this by only emitting the <br> if there is a revnumber AND a
last-update-label.

se-edu/addressbook-level3