StdioClientTransportOptions.EnvironmentVariables should allow nullable values

[AArnott]

The ProcessStartInfo.Environment property allows null values for environment variables. I'm not sure what this means but I suppose/hope it can omit an environment variable from the parent process's environment block rather than define it explicitly with an empty string value.

ModelContextProtocol should allow it too, whatever the semantic is. But it doesn't:

csharp-sdk/src/ModelContextProtocol/Client/StdioClientTransportOptions.cs

Line 48 in 4a8b7fd

public Dictionary<string, string>? EnvironmentVariables { get; set; }