fix: handle None vector in Qdrant update to prevent PointStruct validation error

[themavik]

Summary

Fixes #3780

When a memory action event is NONE (content doesn't need updating), the update() method in mem0/vector_stores/qdrant.py is called with vector=None to only update metadata/payload. Creating a PointStruct with vector=None causes a Pydantic validation error:

6 validation errors for PointStruct
vector.list[float]
  Input should be a valid list [type=list_type, input_value=None, input_type=NoneType]

Root Cause

The update() method unconditionally creates a PointStruct(id=vector_id, vector=vector, payload=payload) regardless of whether vector is None. Qdrant's PointStruct requires vector to be a valid list of floats.

Changes

• When vector is provided: creates PointStruct and upserts as before
• When only payload is provided (vector=None): uses client.set_payload() for a metadata-only update
• When neither is provided: no-op (nothing to update)

Risk Assessment

Low - The fix is additive, using Qdrant's built-in set_payload() method for payload-only updates, which is the correct API for this use case.