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

[Bart-Westenenk]

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.