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 ;).