JVMCBC-39: Set passwords to empty if null.

This change makes sure that if null is passed in for the bucket password,
an empty one is used and the process doesn't stop working.

Change-Id: I4888bbe5e6e8c431395469b94b13b6c8ffd13812
Reviewed-on: http://review.couchbase.org/41482
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>