Fix null gender crash when importing replacement forms

[JohnMwashuma]

Summary

• Fix AttributeError: 'NoneType' object has no attribute 'upper' when importing result forms CSV that contains replacement form rows (empty center_code, station_number, gender)
• Fix find_missing_center_codes SQL query to exclude NULL/empty center codes, which are expected for replacement forms

Changes

• import_result_forms.py:158 - Add null guard before calling .upper() on gender field value
• utils.py:301-306 - Add IS NOT NULL and != '' filters to find_missing_center_codes query so replacement form rows are not flagged as missing centers

Test plan

• Added test fixture CSV with replacement form rows (empty center_code, station_number, gender, name)
• Added test_async_import_result_forms_with_replacements verifying:
  • Import succeeds without error
  • 2 replacement forms created with is_replacement=True
  • Replacement forms have center=None, gender=None, station_number=None
• All 765 existing tests pass with no regressions

Closes #550

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.19%. Comparing base (a20ac93) to head (effc720).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #551   +/-   ##
=======================================
  Coverage   84.18%   84.19%           
=======================================
  Files         165      165           
  Lines       11452    11452           
=======================================
+ Hits         9641     9642    +1     
+ Misses       1811     1810    -1     

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