fix #151 (vendor): prevent writing "None" when agent_notes is None

[Jean-Regis-M]

Summary

Fixes #151 update_vendor_agent_notes writing the literal string "None" when agent_notes=None (test case VND-NOTES-004).

Problem

When called with agent_notes=None, the function builds new_notes = f"{existing_notes}\n\n{agent_notes}".
Python converts None to "None", resulting in "\n\nNone" being appended to the vendor’s notes, corrupting the field.

Root Cause

No guard against None before the f‑string interpolation. The function assumes agent_notes is always a string, but the call site sometimes passes None.

Solution

Add an early return after the vendor is fetched and before any string manipulation:

if agent_notes is None:
    return vendor.to_dict()