File.Exists() is not null when true

[iSazonov]

Addresses a scenario like:

    class Program
    {
        static async Task Main(string[] args)
        {
            if (args.Length != 1)
            {
                Console.WriteLine("...");
                return;
            }

            var inputPath = args[0]; // inputPath is defined as 'string?' and could be null.

            if (!File.Exists(inputPath))
            {
                Console.WriteLine("...");
                return;
            }

            // Since the file exists the inputPath is not null and no compiler warnings should be issued.
            using var inputFile = new StreamReader(inputPath);

            ...
        }
    }

[stephentoub]

Thanks.