fix: update default behavior of expand_env_vars to return empty string for missing env vars

[frostming]

Signed-off-by: Frost Ming [email protected]

Pull Request Checklist

• A news fragment is added in news/ describing what is new.
• Test cases added for changed code.

Describe what you have changed in this PR.

Fix #3653

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.07%. Comparing base (bd8c5d3) to head (f13aa79).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3655   +/-   ##
=======================================
  Coverage   86.07%   86.07%           
=======================================
  Files         119      119           
  Lines       12353    12353           
  Branches     2052     2052           
=======================================
  Hits        10633    10633           
  Misses       1140     1140           
  Partials      580      580           

Flag	Coverage Δ
unittests	85.93% <100.00%> (ø)

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