wrong result for .is_integral() in quadratic field

<div id="comment:0"></div>


```sage
sage: K.<t> = QuadraticField(-10007^2)
sage: a = t / 10007
sage: a.minpoly()
x^2 + 1
sage: a.minpoly() in ZZ['x']
True
sage: a.is_integral()
False
```

Component: **number fields**

_Issue created by migration from https://trac.sagemath.org/ticket/34800_

