JCBC-338: Don't allow null for bucket and password.

This changeset makes sure that the code "fails fast" and throws
a semantically correct exception in case null (or an empty bucket
name) is passed in. Otherwise, the data goes through and throws a
NPE exception down the stack.

Change-Id: I6d73f5dd551d1d9921a25cae8c58783e5f63afed
Reviewed-on: http://review.couchbase.org/27790
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>