docker: don't recreate containers on empty-string property PUTs

Web clients serialize empty form fields as "" while unset values are
stored as None on the node. The bare != diff in the update handler then
sees a phantom change on every full PUT and recreates the container for
nothing -- even when the user only changed a controller-only field such
as netmiko_device_type.

Normalize at the schema boundary ("" -> None for start_command,
environment and extra_hosts; "" -> "/" for console_http_path), make
the setters apply the same canonicalization, and create nodes through
the setters instead of bypassing them in __init__ so both paths store
identical values.

tests/api/routes/compute/test_docker_nodes.py

@@ -306,6 +306,45 @@ class TestDockerNodesRoutes:
         assert response.json()["environment"] == "GNS3=1\nGNS4=0"
         assert response.json()["extra_hosts"] == "test:127.0.0.1"
 
+    async def test_docker_update_empty_strings_do_not_recreate_container(
+            self,
+            app: FastAPI,
+            compute_client: AsyncClient,
+            compute_project: Project
+    ) -> None:
+        """
+        Web clients serialize empty form fields as "" while unset values are
+        stored as None on the node: a full PUT must not see a phantom change
+        and recreate the container for nothing.
+        """
+
+        params = {"name": "DOCKER-EMPTY", "image": "nginx", "environment": ""}
+        with asyncio_patch("gns3server.compute.docker.Docker.list_images", return_value=[{"image": "nginx"}]):
+            with asyncio_patch("gns3server.compute.docker.Docker.query", return_value={"Id": "8bd8153ea8f5"}):
+                response = await compute_client.post(
+                    app.url_path_for("compute:create_docker_node", project_id=compute_project.id), json=params
+                )
+        assert response.status_code == status.HTTP_201_CREATED
+        assert response.json()["environment"] is None  # "" normalized at creation
+        assert response.json()["console_http_path"] == "/"
+        node_id = response.json()["node_id"]
+
+        with asyncio_patch("gns3server.compute.docker.docker_vm.DockerVM.update") as mock:
+            response = await compute_client.put(
+                app.url_path_for("compute:update_docker_node", project_id=compute_project.id, node_id=node_id),
+                json={
+                    "name": "DOCKER-EMPTY",
+                    "start_command": "",
+                    "environment": "",
+                    "extra_hosts": "",
+                    "console_http_path": "",
+                },
+            )
+        assert response.status_code == 200
+        assert not mock.called  # no real change: the container must not be recreated
+        assert response.json()["start_command"] is None
+        assert response.json()["console_http_path"] == "/"
+
 
     async def test_docker_start_capture(self, app: FastAPI, compute_client: AsyncClient, vm: dict) -> None: