docker_registry.lib.index.db: Support empty-query searches

Avoid building queries like:

    SELECT
      repository.id AS repository_id,
      repository.name AS repository_name,
      repository.description AS repository_description
    FROM repository
    WHERE repository.name LIKE %% OR repository.description LIKE %%

When folks don't specify a query term.

Reported-by: Don Laidlaw <don@donlaidlaw.ca>

Fixes #719, but I haven't tested it ;).