[chore] skip stable module check if unset

[songy23]

From https://github.com/open-telemetry/opentelemetry-collector/blob/main/docs/release.md#releasing-opentelemetry-collector:

If not intending to release stable modules, do not specify a version for Release candidate version stable.

validate-versions workflow fails on empty version right now, it should instead skip if stable module candidate version is empty.

[jackgopack4]

tested locally and this works. Ran into this issue recently testing changes to the build/release process for ocb, as well.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.17%. Comparing base (fb5b1e6) to head (f57778c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10734   +/-   ##
=======================================
  Coverage   92.17%   92.17%           
=======================================
  Files         401      401           
  Lines       18786    18786           
=======================================
  Hits        17316    17316           
  Misses       1110     1110           
  Partials      360      360           

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[codeboten]

This makes me wonder if we need the same check for beta values as well... i can imagine we may want to release stable and beta separately in the future

[songy23]

I filed a tracking issue to further fix this script as there are different scenarios to consider
#10735