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There is no 0 variable because there is no difference between 0 and -0. But python lists start with index 0. If you want the value of the variable 2, you will type:

sage: cms()[2]
True

Without the None at the beginning (standing for the value of the 0 variable that does not exist), you will get False (the value of the variable 3).

There seems not be any on-line documentation, but you can actually read it from the sage command line, if you type:

sage: cms?

You will read

OUTPUT:
* If this instance is SAT: A tuple of length "nvars()+1" where the
 "i"-th entry holds an assignment for the "i"-th variables (the
 "0"-th entry is always "None").

and

First, we do not assume anything, note that the first entry is
"None" because there is not "0"-th variable: