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

```diff @@ 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](https://app.codecov.io/gh/3YOURMIND/django-migration-linter/pull/258?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [django\_migration\_linter/sql\_analyser/base.py](https://app.codecov.io/gh/3YOURMIND/django-migration-linter/pull/258?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-ZGphbmdvX21pZ3JhdGlvbl9saW50ZXIvc3FsX2FuYWx5c2VyL2Jhc2UucHk=) | `100.00% <ø> (ø)` | | | [tests/unit/test\_sql\_analyser.py](https://app.codecov.io/gh/3YOURMIND/django-migration-linter/pull/258?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-dGVzdHMvdW5pdC90ZXN0X3NxbF9hbmFseXNlci5weQ==) | `100.00% <100.00%> (ø)` | |

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