bsdinstall: Fix netconfig script when no interfaces are present

The script uses [ -z "$INTERFACES" ] to check if the list of interfaces
is empty and will exit early if so, but INTERFACES always contains at
least a space due to the way it appends the list of wireless devices.
Fix this by only adding the space when there are devices to append,
mirroring the behaviour for non-wireless devices above (both will result
in a redundant leading space when the list is non-empty, but that one is
harmless).

Fixes: 159ca5c844cd ("Adapt to new wireless scheme where base wlan interfaces do not show up in ifconfig anymore.")
MFC after: 1 week

(cherry picked from commit b809c7d6a26924ac351e49a15011da718cc3feec)