push(x) after push(null) should cause an error

Nobody has claimed this yet.

Similarly to #44, the following test currently fails:

```javascript
tape('push after push(null) should cause an error', function (t) {
t.plan(5)
const s = new Readable()
s.on('data', function (data) {
t.equals(data, 'a')
})
s.on('error', function () {
t.pass('stream.push() after EOF')
})
s.on('close', function () {
t.end()
})
t.equals(s.push('a'), true)
t.equals(s.push(null), false)
t.equals(s.push('b'), false)
})
```

Both pushed data statements are passed through and no error like `stream.push() after EOF` as this is an incompatibility to the current Node.js streams and am wondering: is that intentional? should that be documented?

Start with the failing test shown in the issue and compare its push(null) sequence with Node.js stream behavior. Inspect the stream implementation and existing EOF-related tests; done means the intended post-EOF behavior is established and the matching test or documentation is updated.

Written by the indexing model from the issue text.