Fix dimension of C_1 matrix in no-constraint case

The C_1 matrix is of shape o x n, where o is the number of generalized
speeds and n is the number of generalized coordinates (including any
dependent speeds or dependent coordinates). Using the literal 0 results
in matrix size mismatches on operations involving C_1 in cases where
there are no explicit constraints present (i.e., when m == 0).
From the above if statement, the only way to get here is if `n == 0`. I'm not sure if this makes here then.