present-proof v2 (DIF) crashes when input_descriptor.schema is omitted (NoneType uri_groups)

[nodlesh]

ACA‑Py throws a 500 when creating a DIF presentation if the request’s input_descriptors omit schema (schema is optional per Presentation Exchange). The handler dereferences schemas.uri_groups when schemas is None, causing a crash. This blocks send-presentation even when a matching credential is selected by record id.

Environment:

• ACA‑Py version: 1.4.0 (from /status)
• Protocol: present-proof v2.0
• Format: DIF Presentation Exchange
• LDP VC (W3C v2) with Ed25519Signature2020
• Askar (wallet.type=askar, wallet.storage_type=default).

Steps to Reproduce:

1. Issue an LDP VC to the holder.
2. Verifier sends a v2 proof request with DIF definition without schema in input_descriptors (constraints only):

{
  "presentation_definition": {
    "name": "Ayra Business Card LDP",
    "format": { "ldp_vp": { "proof_type": ["Ed25519Signature2020"] } },
    "input_descriptors": [
      {
        "id": "ayra-business-card",
        "purpose": "Must be an Ayra Business Card with Ed25519Signature2020",
        "constraints": {
          "fields": [
            { "path": ["$.type","$.vc.type","$.credential.type"], "filter": { "type": "array", "contains": { "const": "AyraBusinessCard" } } },
            { "path": ["$.proof.type","$.proof[0].type"], "filter": { "type": "string", "const": "Ed25519Signature2020" } }
          ]
        }
      }
    ]
  }
}

3. Holder calls:

   • GET /present-proof-2.0/records/{pres_ex_id}/credentials (returns candidates)

   • POST /present-proof-2.0/records/{pres_ex_id}/send-presentation with record IDs:

     { "auto_remove": false, "dif": { "record_ids": { "ayra-business-card": ["<record_id>"] } } }

Expected:
ACA‑Py should treat missing schema as “no schema filter” and proceed with constraint matching, or at least return a validation error, not crash.

Actual:
send-presentation returns 500, stack trace shows null dereference:
AttributeError: 'NoneType' object has no attribute 'uri_groups' File ".../present_proof/dif/pres_exch_handler.py", line 1014, in filter_schema credential=credential, filter=schemas.uri_groups

Logs (from our run):

• Request payload shows input_descriptors without schema:presentation_definition ... input_descriptors ... constraints ...
• Crash + 500:
  • AttributeError: 'NoneType' object has no attribute 'uri_groups'
  • POST /present-proof-2.0/records//send-presentation → 500

Notes:

• This looks like a null‑check bug in filter_schema when schemas is absent.
• Presentation Exchange allows schema omission; constraints alone should be valid.
• With schema present, the handler does not crash (different issue: schema mismatch).

Workaround (not yet confirmed): include a schema array in each input_descriptor (e.g., W3C VC v2 + Ayra context) so filter_schema has non‑null data.
A safe workaround may also be contained in #3441