FIX: count attacks without converters as no_converters in analytics

[Utkarsh Bahuguna (u7k4rs6)]

Fixes #3088.

Identifiers leave out an empty converter list, so a real attack with no converters had no list and no edges, and its converter array came out NULL, which is "missing". The converter source now falls back to [] when the attack identifier is recorded (the joined attack hash is there), which is "no_converters". Results with no attack identifier stay missing, and a retained list or edges still win first.

Both arrays (with and without the embedded list) get the fallback, so grouping, filters, facets and drill-downs all agree.

Test runs real PromptSendingAttacks, one plain and one with Base64 on request and response, plus a result with no identifier, for both directions:

• group by converter_type gives no_converters / base64converter / missing, one each
• filtering on no_converters returns the plain attack
• get_attack_results_async(has_converters=False) agrees

It fails on main. tests/unit/memory, analytics, backend and models: 6476 passed.