Update flatten.ts

[iegik]

To avoid situation where provided object do not have prototype we should use Object.hasOwn.

Test1:

objToCssArray(Object.create(null))

Test2:

const obj = Object.create(null);
obj.color = 'red';
objToCssArray(obj)

Test3:

const obj = {
  hasOwnProperty: () => { /*...*/ return true },
}
objToCssArray(obj)

[quantizor]

This change unfortunately regresses hot path performance by a decent amount