Allow empty filter group in QueryBuilder

[code28]

This issue is a reminder to allow empty filter groups in QueryBuilder, which was fixed for Vapor 3 in vapor/fluent#616.

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When using the QueryBuilder and a filter group, the resulting SQL statement should be correct, even if no filters are provided.

The usage

User.query(on: req).group(.or) { query in
    // No filters here...
}.all()

Expected behavior

No filters, so no restrictions on the query, so the statement should be:
SELECT * FROM "User" WHERE [].