ArgumentNullException: Value cannot be null, for anonymous volume mounts (#426)

* Dont check if path starts with null

* Check SourceVolumePath not MountVolume obj

* Prefer string.IsNullOrEmpty

Author: David Kale

src/Runner.Worker/Handlers/StepHost.cs

@@ -110,9 +110,9 @@ public string ResolvePathForStepHost(string path)
 
             // try to resolve path inside container if the request path is part of the mount volume
 #if OS_WINDOWS
-            if (Container.MountVolumes.Exists(x => path.StartsWith(x.SourceVolumePath, StringComparison.OrdinalIgnoreCase)))
+            if (Container.MountVolumes.Exists(x => !string.IsNullOrEmpty(x.SourceVolumePath) && path.StartsWith(x.SourceVolumePath, StringComparison.OrdinalIgnoreCase)))
 #else
-            if (Container.MountVolumes.Exists(x => path.StartsWith(x.SourceVolumePath)))
+            if (Container.MountVolumes.Exists(x => !string.IsNullOrEmpty(x.SourceVolumePath) && path.StartsWith(x.SourceVolumePath)))
 #endif
             {
                 return Container.TranslateToContainerPath(path);