test: simplify unit tests when the endpoint returns no content

tests/integration/study_data_blueprint/test_st_storage.py

@@ -245,7 +245,7 @@ def test_lifecycle__nominal(
             json=[siemens_battery_id],
         )
         assert res.status_code == 204, res.json()
-        assert res.text in {"", "null"}  # Old FastAPI versions return 'null'.
+        assert not res.text
 
         # If the short-term storage list is empty, the deletion should be a no-op.
         res = client.request(
@@ -255,7 +255,7 @@ def test_lifecycle__nominal(
             json=[],
         )
         assert res.status_code == 204, res.json()
-        assert res.text in {"", "null"}  # Old FastAPI versions return 'null'.
+        assert not res.text
 
         # It's possible to delete multiple short-term storages at once.
         # In the following example, we will create two short-term storages:
@@ -315,7 +315,7 @@ def test_lifecycle__nominal(
             json=[siemens_battery_id, grand_maison_id],
         )
         assert res.status_code == 204, res.json()
-        assert res.text in {"", "null"}  # Old FastAPI versions return 'null'.
+        assert not res.text
 
         # The list of short-term storages should be empty.
         res = client.get(