[DDC-2235] Fix for using a LEFT JOIN onto an entity with single table inheritance

Possible fix for the bug DDC-2235. I'd love to hear some opinions on whether this is the right way to go about this issue. I'm not particularly familiar with the internals of doctrine so there may be a better solution.

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The issue is when using DQL to perform a left join on an entity using single
table inheritance, doctrine tries to insert an `IN()` predicate into the `WHERE`
clause for all of the discriminator values. That makes sense and is valid, so
it would be wrong to remove that behaviour.

However when using a *left* join having an `IN()` in the main where clause makes
the `LEFT JOIN` pretty much useless, as it implicitly creates a `WHERE NOT NULL`
clause. This commit attempts to fix that by including an `OR IS NULL` in the
query if the join is a `LEFT JOIN`.

I've added some regression tests to ensure this bug never creeps back in. They fail on master (highlighting the bug) and pass after these commits have been applied. I've also included a couple of other queries as tests to be sure only this one case has been affected.