[#6464](https://github.com/doctrine/doctrine2/pull/6464)
and [#6475](https://github.com/doctrine/doctrine2/pull/6475).
Specifically, a dangling undefined variable remained in the
code, causing some `SqlWalker` functionality to misbehave.
The undefined variable wasn't noticed due to it being placed
in an `empty()` expression.

Total issues resolved: **1**

- [6621: Fixed usage of undefined variable in `empty()` expression in `SqlWalker`](https://github.com/doctrine/doctrine2/pull/6621) thanks to @Powerhamster