Avoid setting the templated 'Number' to '0' (double) in trace(Tensor<2,dim,Number>)

Enables using trace also with e.g. `Tensor<2,dim,VectorizedArray<double> >.`
From an equation point of view you are right. But this line produces a sparse matrix that is matched against the equivalent matrix-free operation in line 88 above (velocity.get_symmetric_gradient() got replaced by velocity.get_gradient()), which is the purpose of this test and fits with my change.