SubstreamPartitionRouter emits partitions for explicitly null parent_key values, producing literal 'None' in request paths

Start in airbyte_cdk/sources/declarative/partition_routers/substream_partition_router.py around lines 215-239, then use the minimal reproduction with an explicitly null parent_key to trace the emitted StreamSlice. Done means null partition values are skipped like missing paths, preventing a request path containing literal None; preserve the existing behavior for valid values.

Symptom

SubstreamPartitionRouter handles a missing parent_key path and an explicitly null parent_key value inconsistently:

• parent record LACKS the parent_key path >> dpath.get raises KeyError >> the record is silently skipped (emit_slice = False), no partition is produced;
• parent record HAS the path but its value is explicitly null >> dpath.get returns None >> a partition IS produced with a None partition value.

A downstream requester that interpolates the partition value into its path then issues a garbage request with the literal string None in the URL, e.g. GET https://api.linkedin.com/rest/posts/None. Most APIs answer 400, which maps to FAIL via DEFAULT_ERROR_MAPPING and kills the whole sync.

Root cause

At tag v7.23.8, airbyte_cdk/sources/declarative/partition_routers/substream_partition_router.py:

• lines 232-239: only the KeyError branch sets emit_slice = False (with the in-code FIXME noting the missing log);
• line 215-216: the guard only checks parent_record is not None, not the extracted partition_value.

So partition_value = None flows through to the yielded StreamSlice, and Jinja stringifies it during request interpolation ('None').

Minimal reproduction

Parent stream returns {"id": "c1", "content": {"reference": null}}; child stream config:

partition_router:
  type: SubstreamPartitionRouter
  parent_stream_configs:
    - type: ParentStreamConfig
      parent_key: content/reference
      partition_field: post_urn
      stream: { $ref: "#/definitions/streams/parent" }

with requester path: posts/{{ stream_slice.get('post_urn') }}. Observed request: GET <url_base>/posts/None. A parent record with no content key at all is correctly skipped.

Expected

A null partition value should be treated like a missing one (skip the record, ideally with a log line), or at minimum be skippable via configuration. Emitting a partition whose value is None is never useful: it cannot address a real resource.

Impact

Any substream whose parent API emits explicit null for the parent key field fails the sync with a confusing 400 on a .../None URL. Connectors have to work around it with response-filter IGNOREs on the child requester, which also masks genuinely malformed requests.

Precedent

Found while reviewing https://github.com/airbytehq/airbyte/pull/81509 (source-linkedin-ads videos stream: creatives.content.reference >> GET /rest/posts/{urn}). The connector-side mitigation there is an IGNORE response filter matching URN-related 400s.