dialects: (linalg) allow for empty results in linalg.fill custom format

[superlopuh]

Fixes #2228: linalg.fill shouldn't have a fixed result

CC @gabrielrodcanal

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.60%. Comparing base (d398151) to head (58663b1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2363   +/-   ##
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  Coverage   89.60%   89.60%           
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  Files         345      345           
  Lines       40691    40691           
  Branches     6050     6050           
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  Hits        36462    36462           
  Misses       3352     3352           
  Partials      877      877           

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[Moxinilian]

I wish there was some constraints on whether the optional result is there or not but I guess even MLIR does not do that...