Move zero-sized transfer dirs out of the way

Otherwise AutoTx will try to process them in each iteration but fail
right away, thus blocking all other transfers while the dir is there.

Fixes #49

ATxService/RoboCommand.cs

@@ -26,6 +26,11 @@ namespace ATxService
             if (totalSize == 0) {
                 Log.Warn("Total size of all files in [{0}] is zero, NOT STARTING a transfer!",
                     sourcePath);
+                // we also have to move it out of the way, otherwise it will block all transfers:
+                var sourceDir = new DirectoryInfo(sourcePath);
+                var errorTargetPath = Path.Combine(_config.ErrorPath, TimeUtils.Timestamp());
+                sourceDir.MoveTo(errorTargetPath);
+                Log.Warn("Moved empty directory tree [{0}] to [{1}].", sourcePath, errorTargetPath);
                 return;
             }
             Log.Debug("Size of all files under [{0}] is {1} bytes ({2})",