DDC-2129: [GH-508] Fix and test for DDC-2073

[doctrinebot]

Jira issue originally created by user @beberlei:

This issue is created automatically through a Github pull request on behalf of mnapoli:

Url: #508

Message:

Fix and test for [DDC-2073](http://www.doctrine-project.org/jira/browse/[DDC-2073]%28http://www.doctrine-project.org/jira/browse/DDC-2073%29).

Ticket extract:

PersistentCollection::matching() always returns empty collection, when passed criteria's expression is created using ExpressionBuilder::isNull()
This happens because expression created by ExpressionBuilder::isNull() is wrongly translated to SQL using '=' operator instead of 'IS NULL'.
If the collection is already loaded, result is as expected.

[doctrinebot]

Comment created by mnapoli:

Argh this issue should be closed, I apparently failed linking the pull request to DDC-2073

[doctrinebot]

Comment created by @beberlei:

A related Github Pull-Request [GH-508] was closed
#508

[doctrinebot]

Issue was closed with resolution "Fixed"

[doctrinebot]

Comment created by @doctrinebot:

A related Github Pull-Request [GH-508] was closed:
doctrine/dbal#508