Fix some warnings in C#

[bfops]

Description of Changes

This comes up when doing a fresh dotnet test, which causes dotnet test -warnaserror to fail in CI:

➜ dotnet test -warnaserror
  Determining projects to restore...
  Restored /home/work/SpacetimeDBPrivate/public/crates/bindings-csharp/Runtime/Runtime.csproj (in 126 ms).
  Restored /home/work/SpacetimeDBPrivate/public/crates/bindings-csharp/BSATN.Runtime/BSATN.Runtime.csproj (in 133 ms).
  Restored /home/work/SpacetimeDBPrivate/public/crates/bindings-csharp/BSATN.Runtime.Tests/BSATN.Runtime.Tests.csproj (in 142 ms).
  Restored /home/work/SpacetimeDBPrivate/public/crates/bindings-csharp/BSATN.Codegen/BSATN.Codegen.csproj (in 148 ms).
  Restored /home/work/SpacetimeDBPrivate/public/crates/bindings-csharp/Codegen/Codegen.csproj (in 148 ms).
  Restored /home/work/SpacetimeDBPrivate/public/crates/bindings-csharp/Codegen.Tests/Codegen.Tests.csproj (in 280 ms).
  BSATN.Codegen -> /home/work/SpacetimeDBPrivate/public/crates/bindings-csharp/BSATN.Codegen/bin/Debug/netstandard2.0/SpacetimeDB.BSATN.Codegen.dll
  BSATN.Runtime -> /home/work/SpacetimeDBPrivate/public/crates/bindings-csharp/BSATN.Runtime/bin/Debug/netstandard2.1/SpacetimeDB.BSATN.Runtime.dll
  Codegen -> /home/work/SpacetimeDBPrivate/public/crates/bindings-csharp/Codegen/bin/Debug/netstandard2.0/SpacetimeDB.Codegen.dll
/home/work/SpacetimeDBPrivate/public/crates/bindings-csharp/BSATN.Runtime/QueryBuilder.cs(880,9): error CA1510: Use 'ArgumentNullException.ThrowIfNull' instead of explicitly throwing a new exception instance (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules/ca1510) [/home/work/SpacetimeDBPrivate/public/crates/bindings-csharp/BSATN.Runtime/BSATN.Runtime.csproj::TargetFramework=net8.0]
/home/work/SpacetimeDBPrivate/public/crates/bindings-csharp/BSATN.Runtime/Builtins.cs(3,1): error IDE0005: Using directive is unnecessary. (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/style-rules/ide0005) [/home/work/SpacetimeDBPrivate/public/crates/bindings-csharp/BSATN.Runtime/BSATN.Runtime.csproj::TargetFramework=net8.0]
  BSATN.Runtime -> /home/work/SpacetimeDBPrivate/public/crates/bindings-csharp/BSATN.Runtime/bin/Debug/net8.0/SpacetimeDB.BSATN.Runtime.dll
  Codegen.Tests -> /home/work/SpacetimeDBPrivate/public/crates/bindings-csharp/Codegen.Tests/bin/Debug/net8.0/Codegen.Tests.dll

The fix is done by AI.

API and ABI breaking changes

None. Behavior should be the same.

Expected complexity level and risk

1

Testing

git clean -fdx . && dotnet test -warnaserror no longer fails

[rekhoff]

I have not tested this, but the changes look good to me.