Of course, it is silent. What would induce such thinkers to introduce a poison pill? There is no union with conditional ratification. Madison made that clear with his comment that once in the union always in the Union. Washington warned against secession in his Farewell Address which had been worked on by both Madison and Hamilton. This address has been misrepresented as Isolationist when it is anti-secession.
With Congressional approval secession might be legal but that would require a lot more thought than I choose to give it.
The US Constitutions would not have been ratified if no secession clause ever article had been included. Secession is not unconstitutional.