Add check for zero monomial in monomial_coefficient

[keltecc]

Fixes #39255

Added check for zero monomial and corresponding test case.

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• I have created tests covering the changes.
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[fchapoton]

does it work with "is NULL" ?

there is no final dot at the end of error messages (it's a Python general convention)

[keltecc]

Thank you for review, fixed.

does it work with "is NULL" ?

It works.

there is no final dot at the end of error messages (it's a Python general convention)

I tried to keep consistency with another error message in this function:

        if mon._parent is not self._parent:
            raise TypeError("mon must have same parent as self.")

[tscrim]

Thanks. Positive review.