discriminant throwing an error when it probably shouldn't

typical example:
```
i1 : R=ZZ/2[x]

o1 = R

o1 : PolynomialRing

i2 : F=x^24+x^22+x^20+x^14+x^12+x^8+x^4

24 22 20 14 12 8 4
o2 = x + x + x + x + x + x + x

o2 : R

i3 : discriminant(F,x)
stdio:3:12(3): error: expected nonzero polynomials
```
you can replace `ZZ/2` with any other `ZZ/p` and exponents divisible by `p`.

Start by reproducing the provided discriminant(F,x) example in Macaulay2, then compare it with the cases over ZZ/p described in the issue. Trace the discriminant entry point and its handling of polynomials whose exponents are divisible by p; done means these examples no longer raise the unexpected “expected nonzero polynomials” error.

Written by the indexing model from the issue text.