[bug] Nil concatenation in codeium/api.lua output handler

[Mutarov]

Describe the bug

When Codeium server returns unexpected nil output, error occurs:
attempt to concatenate local 'v' (a nil value)

Steps to reproduce

1. Use Neovim with windsurf.nvim
2. Trigger Codeium completion
3. Simulate nil output

fixed in #328