Don't consider partial index creation as making a column not nullable.

[David-Wobrock]

Fixes #250

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (366d16b) 93.94% compared to head (02cabab) 93.95%.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #258   +/-   ##
=======================================
  Coverage   93.94%   93.95%           
=======================================
  Files          77       77           
  Lines        1966     1969    +3     
=======================================
+ Hits         1847     1850    +3     
  Misses        119      119           

Impacted Files	Coverage Δ
django_migration_linter/sql_analyser/base.py	100.00% <ø> (ø)
tests/unit/test_sql_analyser.py	100.00% <100.00%> (ø)

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