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).
I think the logic is a bit convoluted but still should be correct.  Sympy matrices can be 0xn or nx0 so it should still work.  The issue I ran into was on line 322:

r1c1 += (A_qu * C_1)

If C_1 is simply 0, then the product above is the zeros(n, o), rather than zeros(n, n).  This causes the __add__ operation to fail do to the fact that r1c1 is of shape (n, n).