otelslog: Transform nil attribute to empty log.Value

[pellared]

Fixes #6242

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.9%. Comparing base (32c279a) to head (6a15ea4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #6246   +/-   ##
=====================================
  Coverage   66.9%   66.9%           
=====================================
  Files        190     190           
  Lines      12538   12540    +2     
=====================================
+ Hits        8393    8395    +2     
  Misses      3855    3855           
  Partials     290     290           

Files with missing lines	Coverage Δ
bridges/otelslog/handler.go	96.4% <100.0%> (+<0.1%)	⬆️