Fix falseyness of hashes/maps.

Hashes and maps are falsey only if they are empty. The current test
deemed QVariantHashes/-Maps as false if .toString().isEmpty(), which
is always true. This patch makes the test use the QHash::isEmpty()
and QMap::isEmpty() instead.

This fixes the "Context" tests in sections.json and inverted.json
(and also by "accident" the "Inverted Section" test in ~lambdas.json.
I'll fix so that the test doesn't even load tests from ~lambdas.json
in a separate commit.
Merge pull request #10 from estan/fix-hash-map-falsyness

Fix falseyness of hashes/maps.