chore: enable nullable compiling option

[coveralls]

Pull Request Test Coverage Report for Build 6460281585

• 59 of 80 (73.75%) changed or added relevant lines in 14 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.8%) to 85.936%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
DuckDB.NET.Data/DuckDBCommand.cs	14	15	93.33%
DuckDB.NET.Data/DuckDBParameter.cs	7	8	87.5%
DuckDB.NET.Data/Internal/ConnectionReference.cs	0	1	0.0%
DuckDB.NET.Data/VectorDataReader.cs	5	6	83.33%
DuckDB.NET.Data/Internal/ConnectionManager.cs	6	8	75.0%
DuckDB.NET.Data/DuckDBDataReader.cs	6	9	66.67%
DuckDB.NET.Data/DuckDBConnection.cs	10	22	45.45%
<!--	Total:	59	80	73.75%	-->

Totals
Change from base Build 6455590535:	-1.8%
Covered Lines:	899
Relevant Lines:	1023

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💛 - Coveralls

[Giorgi]

I updated GetList not to use dynamic anymore. Can you resolve the conflicts? Thanks

[Seddryck]

PR refreshed with latest commits from your side

[Giorgi]

Merged, thanks @Seddryck