Ratification only requires two-thirds of the “members present”, not two-thirds of the members.
There may be quorum requirements in the Senate’s rules, but I’m not certain on that.
There is a quorum rule: 51 must be present making 34, 2/3, a passing vote. However, since when has Obama gave a damn about the law or the Constitution? I suspect he’ll just have a few Senators over and simply declare the treaty as having been passed.