Fix unnecessary Docker container recreation when renaming a project

When renaming a project that has running Docker containers, the containers
were unnecessarily stopped, removed, and recreated, even though the project
name change doesn't affect container configuration.

Root cause:
- Client sends complete project object including variables: [] during rename
- Controller unconditionally notified all computes about the update
- Docker nodes rebuild containers on any project update notification

Solution:
- Only notify compute nodes when variables field has actual content
- Treat None and [] as semantically equivalent (no variables)
- Empty variables don't affect running containers, so no need to update

Impact:
- Project rename operations no longer trigger ~7 second container rebuilds
- Only actual variable changes trigger container recreation
- Fixes issue #2760

gns3server/controller/project.py

@@ -197,9 +197,11 @@ class Project:
             self.emit_controller_notification("project.updated", self.asdict())
             self.dump()
 
-            # update on computes
-            for compute in list(self._project_created_on_compute):
-                await compute.put(f"/projects/{self._id}", {"variables": self.variables})
+            # Only notify computes if variables actually changed and have content
+            # None and empty list are semantically equivalent (no variables) and don't affect running nodes
+            if "variables" in kwargs and kwargs["variables"]:
+                for compute in list(self._project_created_on_compute):
+                    await compute.put(f"/projects/{self._id}", {"variables": self.variables})
 
     def reset(self):
         """