fix: only attempt to mount static files dir (for chatUI) if already generated

[cpacker]

Please describe the purpose of this pull request.

• ChatUI serves static files to support web app
• However, if users grab memgpt via pip install, there's no chatui directory (not bundled atm)
• The current REST API code (run on memgpt server) assumes that the static dirs path already exists
  • So memgpt server is effectively broken in the current pip install

Have you tested this this PR?

Yes, this local diff on the pip install (0.3.1) fixes the problem:

% memgpt server
INFO:     Started server process [62156]
INFO:     Waiting for application startup.
Writing out openapi.json file
INFO:     Application startup complete.
INFO:     Uvicorn running on http://localhost:8283 (Press CTRL+C to quit)

[cpacker]

@goetzrobin in your opinion is this the correct way to fix this issue (at least until chatui is bundled into the pip install too?)