Fix filtering behavior within non-existent @optional blocks.

[obi1kenobi]

Analogous update to #692, just for filtering instead of type coercion.

We used to attempt to apply filters to the contents of an @optional block when the @optional edge did not exist. This is semantically incorrect behavior — it makes the combination of filters and @optional useless since the optional results always get discarded when the @optional edge doesn't exist.

This PR reproduces the problem in a variety of settings, and fixes all of them.

• Add test data that reproduces incorrect query evaluation.
• Add filtering fix and update test cases.