connspec: use default couchbase:// if no string passed.

Otherwise segfaults if a v3 options is passed empty.

Change-Id: I065436c71e3c011732b1faec61041827b24968a8
Reviewed-on: http://review.couchbase.org/38798
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>