Well migration due to the electric field can be ignored in typical setups, it is orders of magnitude slower than the diffusion.
If you look at every derivation of the function describing current vs applied potential in every electrochemical analytical method, be it polarography, voltammetry, CV, whatever, it is only diffusion that is taken into account. Migration is always ignored, as it doesn't matter.
It starts to being observable at very high potentials (Wien effect). But we are talking about gradients in the range of at least MV per cm.