fix subtle issue with settings env variables

[samuelcolvin]

Change Summary

Fix issue with BaseSettings inheritance and alias getting set to None

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)

[codecov]

Codecov Report

Merging #882 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #882   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          16     16           
  Lines        2716   2716           
  Branches      518    518           
=====================================
  Hits         2716   2716

Impacted Files	Coverage Δ
pydantic/fields.py	100% <100%> (ø)	⬆️

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