Minority Carrier Diffusions Equations in the derivation of Shockley equation
Since the electrical field is nul by assumption.


No there is an electric field.
There is a balance in the depletion region, a built in E field
(from the n and p junction, that you can change with an external bias voltage)
that drives charge carriers one way,
and diffusion the other.

For me the current due to diffusion is harder to understand.