Support nullable as optional dependency

[skarllot]

I'm getting problems with optional dependencies as defined by LoggerFactory and ConsoleLoggerProvider.

public LoggerFactory(IEnumerable<ILoggerProvider> providers, IOptionsMonitor<LoggerFilterOptions> filterOption, IOptions<LoggerFactoryOptions>? options)

public ConsoleLoggerProvider(IOptionsMonitor<ConsoleLoggerOptions> options, IEnumerable<ConsoleFormatter>? formatters)

The source generator generates the following invalid code:

// ...
        private System.Collections.Generic.IEnumerable<Microsoft.Extensions.Logging.Console.ConsoleFormatter>?? _IEnumerable_ConsoleFormatter;
// ...
            if (type == typeof(System.Collections.Generic.IEnumerable<Microsoft.Extensions.Logging.Console.ConsoleFormatter>?)) return this.GetService<System.Collections.Generic.IEnumerable<Microsoft.Extensions.Logging.Console.ConsoleFormatter>?>();
// ...

[pakrym]

Fixed in #147