Merge pull request #558 from skaryshev/skaryshev_msi_fix

InvokeProcessAsync workingDirectory param default to empty string

src/Squirrel/Utility.cs

@@ -177,7 +177,7 @@ public static T Retry<T>(this Func<T> block, int retries = 2)
             }
         }
 
-        public static Task<Tuple<int, string>> InvokeProcessAsync(string fileName, string arguments, CancellationToken ct, string workingDirectory = null)
+        public static Task<Tuple<int, string>> InvokeProcessAsync(string fileName, string arguments, CancellationToken ct, string workingDirectory = "")
         {
             var psi = new ProcessStartInfo(fileName, arguments);
             if (Environment.OSVersion.Platform != PlatformID.Win32NT && fileName.EndsWith (".exe", StringComparison.OrdinalIgnoreCase)) {