IRRdb filters: use "AS<n>" in case of an empty AS-SET from PeeringDB

[pierky]

This issue is used to track the problem raised by @job with #14.

When no AS-SETs are configured for a client nor for its ASN, the AS-SET from the PeeringDB record can be used to build IRR-based filters. Until now, if the AS-SET fetched from PeeringDB is empty/non-existent, also the resulting filter is empty.

[pierky]

With v0.12.3, the ASn macro is always considered when building IRRdb-based filters, so even in the case of empty/non-existent AS-SETs fetched from PeeringDB an attempt is made by expanding the ASN autnum object itself.