Adjusted type hint for '_get_session()'

[tieneupin]

_get_session() can return either None or a Generator object that yields sqlalchemy.orm.Session or None. Updated the type hint to reflect that.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 28.77%. Comparing base (84770ff) to head (c42a158).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #555      +/-   ##
==========================================
- Coverage   28.85%   28.77%   -0.09%     
==========================================
  Files          80       80              
  Lines       10556    10646      +90     
  Branches     1406     1439      +33     
==========================================
+ Hits         3046     3063      +17     
- Misses       7408     7478      +70     
- Partials      102      105       +3     

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[tieneupin]

Same fix was included as part of PR #560 .