Problem with autodiff in multifield

This issue follows the one described [here](https://github.com/gridap/Gridap.jl/issues/913) as concerns the problem to solve but involves another error.

Now the idea is to impose a nonlinear flux between the top line of the mesh on which is defined a "lineic" variable `uᵗ` and the whole domain on which is defined the "bulk" variable `u`. The problem seems to be correctly solved if the jacobian is explicitly provided but an error is raised in case of autodiff resolution.

I attach an example.

[test_multi_nonlinear.jl.txt](https://github.com/gridap/Gridap.jl/files/11926570/test_multi_nonlinear.jl.txt)

Start with the attached test_multi_nonlinear.jl.txt example and compare the explicitly provided Jacobian path with autodiff for the nonlinear flux between the lineic and bulk variables. Reproduce the autodiff error first; done means the example solves successfully with autodiff, matching the working explicit-Jacobian behavior.

Written by the indexing model from the issue text.