A `where` statement with `= [potentially null value]` should not be possible

The following query builder where clause will always evaluate to true, as in SQL `col = null` is always evaluated to true
```
$querybuilder
  ->select('f')
  ->from('foo', 'f')
  ->where('f.bar = :bar')
  ->setParameter('bar', null)
```

However, this may be unexpected as the developer may expect it also filters the null values.