ipfix: allow empty targets column in table IPFIX

ipfix: allow empty targets column in table IPFIX

The "targets" column in IPFIX had a min=1 constraints, so OVSDB
implicitly adds an empty string "" into that column if no value is
given.  No connection can be opened to a target with address "", so
the whole IPFIX exporter for that row was disabled until that ""
target was removed by users.  That behavior is correct but proved to
be unintuitive to users.

This patch removes the min=1 constraint, to avoid the trouble for
users who insert IPFIX rows with no targets: it eliminates the log
messages due to failed connections to target "", and eliminates the
need to manually remove the "" target after row insertion.

This doesn't impact the behavior for any existing row, whether it has
a "" target or not.

Signed-off-by: Romain Lenglet <rlenglet@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>