MB-12056: don't crash on update of bucket with invalid authType

In MB-12056 it looks like second submit is somehow triggered. And that
second submit sees form input disabled which causes post with invalid
authType. The later caused other_buckets_ram_quota property to be
undefined which crashed menelaus_web_buckets:interpret_ram_quota.

We now anticipate this undefinedness which fixes the problem.

Change-Id: Ia2b5b51097a5cab53c36677a441fc0096d69f48b
Reviewed-on: http://review.couchbase.org/40889
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>