Fix mapping activation in view mode (#8687)

Don't wait for differ response if there are no tracings, for example in
view mode.

### URL of deployed dev instance (used for testing):
- https://fixmappingactivationviewmode.webknossos.xyz

### Steps to test:
- Open a dataset in view mode and activate a mapping. Previously, the
mapping activation was stuck and the mapping never got activated.
- Follow the testing instructions in
#8676 to check that the
race condition is still fixed

------
(Please delete unneeded items, merge only when none are left open)
- [x] Updated
[changelog](../blob/master/CHANGELOG.unreleased.md#unreleased)
- [x] Removed dev-only changes like prints and application.conf edits
- [x] Considered [common edge
cases](../blob/master/.github/common_edge_cases.md)

---------

Co-authored-by: Philipp Otto <philipp.4096@gmail.com>

Author: daniel-wer

CHANGELOG.unreleased.md

@@ -19,6 +19,8 @@ For upgrade instructions, please check the [migration guide](MIGRATIONS.released
 ### Fixed
 - Improved efficiency of saving bounding box related changes. [#8492](https://github.com/scalableminds/webknossos/pull/8492)
 - When deleting a dataset, its caches are cleared, so that if a new dataset by the same name is uploaded afterwards, only new data is loaded. [#8638](https://github.com/scalableminds/webknossos/pull/8638)
+- Fixed a race condition when starting proofreading with a split action. [#8676](https://github.com/scalableminds/webknossos/pull/8676)
+- Fixed that activating a mapping got stuck when a dataset was opened in "view" mode. [#8687](https://github.com/scalableminds/webknossos/pull/8687)
 
 ### Removed
 

frontend/javascripts/viewer/model.ts

@@ -370,8 +370,10 @@ export class WebKnossosModel {
         }
       }
 
-      Store.dispatch(ensureTracingsWereDiffedToSaveQueueAction(callback));
-      await deferred.promise();
+      if (tracingIds.size > 0) {
+        Store.dispatch(ensureTracingsWereDiffedToSaveQueueAction(callback));
+        await deferred.promise();
+      }
       return true;
     }