fix(nvim): convert vim.NIL args to nil in codecompanion tools

[Davidyz]

Some LLMs decide to send nulls in tool args, which then get converted to vim.NIL.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.72%. Comparing base (915ad56) to head (72648b8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #298   +/-   ##
=======================================
  Coverage   99.72%   99.72%           
=======================================
  Files          25       25           
  Lines        1838     1838           
=======================================
  Hits         1833     1833           
  Misses          5        5           

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