Only load structures if they are not null
Do not call $result->getNodes() for dbal

Jackalope-doctrine-dbal currently has a bug (fixed in master) which
prevents $result->getNodes() working as expected. This change avoids
calling that method, and has no performance penalty that I am aware of.
Merge branch 'bugfix/avoid_jackalope_dbal_getnodes_issue' into develop