Don't fetch mesh files for segmentation layers without fallback layers (#8726)

The fetching of the meshes was not guarded against segmentation layer
without a fallback. Only segmentation layer with fallback can have
meshes.

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

### Steps to test:
- On master: Create a skeleton only annotaiton
- add a empty volume layer without fallback
- open segmentation tab -> watch it crash
- On this branch: Do the same
- No crash  

### Issues:
- fixes report
https://scm.slack.com/archives/C02H5T8Q08P/p1751282276200889

------
(Please delete unneeded items, merge only when none are left open)
- [x] Added changelog entry (create a `$PR_NUMBER.md` file in
`unreleased_changes` or use `./tools/create-changelog-entry.py`)

Author: MichaelBuessemeyer

frontend/javascripts/viewer/model/sagas/meshes/precomputed_mesh_saga.ts

@@ -58,7 +58,12 @@ let fetchDeferredsPerLayer: Record<string, Deferred<Array<APIMeshFileInfo>, unkn
 function* maybeFetchMeshFiles(action: MaybeFetchMeshFilesAction): Saga<void> {
   const { segmentationLayer, dataset, mustRequest, autoActivate, callback } = action;
 
-  if (!segmentationLayer) {
+  // Only an segmentation layer with an existing fallback layer can have meshfiles.
+  if (
+    !segmentationLayer ||
+    !("fallbackLayer" in segmentationLayer) ||
+    segmentationLayer.fallbackLayer == null
+  ) {
     callback([]);
     return;
   }

unreleased_changes/8726.md

@@ -0,0 +1,2 @@
+### Fixed
+- Fixed a bug when loading mesh files for segmentation layer without fallback data. Such layer cannot have mesh files.