Suppress output of Jacobian

the return value of a cell is shown irrespective of `,`. Make the return value `nothing` to have no output and add `# hide` to not show the `nothing` in the rendered code.

Author: baggepinnen

docs/src/mtkitize_tutorials/sparse_jacobians.md

@@ -51,6 +51,7 @@ Now let's use `modelingtoolkitize` to generate the symbolic version:
 
 ```@example sparsejac
 sys = modelingtoolkitize(prob);
+nothing # hide
 ```
 
 Now we regenerate the problem using `jac=true` for the analytical Jacobian