Release 2.12: 3p3c diffusion is always zero

On the release branch 2.12 the diffusion of 3p3c models is calculated in the local residual. There the molar density from the fluxvariables is used. However, in the 3p3c flux-vars the molar density is only initialized to 0 and never set. Therefor all diffusive fluxes are 0.