fwrite: Improve validation of the na argument

[mcol]

This moves the conversion to character after the stopifnot(), so that we get a consistent error message for both na = data.frame() and na = data.table().

Fixes #7866.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.01%. Comparing base (6793af2) to head (9f32888).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7867   +/-   ##
=======================================
  Coverage   99.01%   99.01%           
=======================================
  Files          88       88           
  Lines       17292    17292           
=======================================
  Hits        17122    17122           
  Misses        170      170           

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