This fixes sagemath#36838. Here, I have added a condition to check whether
mk=0 or not. Because whenever mk=0 we should give mk^(n-i) (i.e. 0^(n-i))
instead of only 0 considering (n-i) can be equal to zero and in this
case 0^(n-i) will be more accurate than only 0.

This change handles all the errors which were occuring in previous
commit and this commit handles the case of mx=0 very effectively.

Created tests covering the changes and checking whether the
trac:`36838` is fixed or not.

Instead of returning 0^(n-i), it would be more precise if
we reutrn value of  kroncker_delta function, it will be helpful when
we try to evaluate the final matrix using the value of n.

Changed the final answer given by test of this PR.

Changed the answer of doctest  according to new changes.

Corrected the doctest and improved some code styles, which were not
correct according to guidelines for python coding for sage.

Updated the old doctests which were failing before and updated the comment
to make it more readable.