[BUG] ChatGPT Import Fails on Conversations with Null Titles

[jrand0m]

Bug Description
ChatGPT importer crashes when encountering conversations with null titles in conversations.json, causing entire import to fail.

Steps To Reproduce

1. Export ChatGPT conversations to conversations.json
2. Ensure some conversations have "title": null in JSON
3. Run bm import chatgpt conversations.json --project Life
4. Import fails with TypeError: expected string or bytes-like object, got 'NoneType'

Expected Behavior

• Skip conversations with null titles with warning message
• Continue importing valid conversations
• Report summary: "X imported, Y skipped"

Actual Behavior

• Entire import fails on first null title
• No conversations imported
• Traceback shown instead of graceful handling

Error Stack

File ".../chatgpt_importer.py", line 97, in _format_chat_content
    clean_title = clean_filename(conversation["title"])
File ".../utils.py", line 18, in clean_filename
    name = re.sub(r"[\s\-,.:/\\\[\]\(\)]+", "_", name)
TypeError: expected string or bytes-like object, got 'NoneType'

Root Cause
clean_filename() doesn't validate input before regex operation

Environment

• OS: macOS
• Python version: 3.12.12
• Basic Memory version: 0.16.2
• Installation method: uv
• Data source: ChatGPT export