Enable 'nullable'

[dev-gavinfang]

for #131

[coveralls]

Pull Request Test Coverage Report for Build 6428318505

• 32 of 49 (65.31%) changed or added relevant lines in 9 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-2.0%) to 80.872%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
DuckDB.NET.Data/DuckDBCommand.cs	7	8	87.5%
DuckDB.NET.Data/Internal/ConnectionReference.cs	0	1	0.0%
DuckDB.NET.Data/Internal/ConnectionManager.cs	5	7	71.43%
DuckDB.NET.Data/DuckDBDataReader.cs	11	24	45.83%
<!--	Total:	32	49	65.31%	-->

Files with Coverage Reduction	New Missed Lines	%
DuckDB.NET.Data/DuckDBConnection.cs	1	91.21%
DuckDB.NET.Data/DuckDBCommand.cs	2	82.81%
<!--	Total:	3		-->

Totals
Change from base Build 6420695333:	-2.0%
Covered Lines:	922
Relevant Lines:	1079

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💛 - Coveralls

[dev-gavinfang]

I'm not sure if adding nullability is worth it because of so many pragma statements.

it's ok. the PR was created when I tried to read the code, maybe it's not a good idea for now : )

[Seddryck]

I think I can submit a PR with some changes. nullable will help in the future to prevent some static mistakes. It's also a good way to pimp up some code from the first commits where some edge cases haven't been implemented.

[Giorgi]

@Seddryck That will be good but keep in mind that I merged another branch with large refactoring and there will probably be many conflicts from this PR.