misc: allow disabling of ipfix exporter via api

misc: allow disabling of ipfix exporter via api

Zero collector address indicates that the IPFIX exporter is disabled.
You can zero the collector address via vppctl but you can't do it via
the API because the handler returns an error if the address is zero.

With this change, do not return an error in the handler when the
collector address is zero thus allowing the IPFIX exporter be
disabled.

Type: fix
Signed-off-by: Alexander Chernavin <achernavin@netgate.com>
Change-Id: I1335c0d06af81f11d743c395399a360c612c0c21