batman-adv: fix visualization output without neighbors on the primary interface

The primary entry and the corresponding secondary entries are missing when there
are no neighbors on the primary interface. This also causes the TT entries to
miss and makes nodes with multiply secondary interface fall apart since there
is no way to see they are related without a primary entry.

Fix this by always emitting a primary entry.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
Signed-off-by: Sven Eckelmann <sven@narfation.org>