When setting password field, check password presence

Fixes segmentation fault in this code

    Couchbase.connect(:username => "default", :bucket => "default")

Change-Id: I4346b41c67cc30f173ef155360381f26aa4d873c
Reviewed-on: http://review.couchbase.org/28578
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>