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

src/Runner.Listener/Configuration/PromptManager.cs

@@ -72,6 +72,10 @@ namespace GitHub.Runner.Listener.Configuration
                 {
                     return defaultValue;
                 }
+                else if (isOptional) 
+                {
+                    return string.Empty;
+                }
 
                 // Otherwise throw.
                 throw new Exception($"Invalid configuration provided for {argName}. Terminating unattended configuration.");

src/Runner.Worker/Handlers/StepHost.cs

@@ -110,9 +110,9 @@ namespace GitHub.Runner.Worker.Handlers
 
             // 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);