[exporter/otlp] fix the validation of unix socket endpoint

[yumosx]

Description

Unix socket endpoints do not have a host:port structure, so port validation should be skipped to prevent unnecessary errors.

Link to tracking issue

Fixes #13826

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.41%. Comparing base (4b4e557) to head (c2fcf0e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #13831   +/-   ##
=======================================
  Coverage   91.40%   91.41%           
=======================================
  Files         646      646           
  Lines       42606    42612    +6     
=======================================
+ Hits        38945    38954    +9     
+ Misses       2838     2836    -2     
+ Partials      823      822    -1     

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