Fix history api with no constraints (#36979)

homeassistant/components/history/__init__.py

@@ -423,7 +423,7 @@ async def get(
         self, request: web.Request, datetime: Optional[str] = None
     ) -> web.Response:
         """Return history over a period of time."""
-
+        datetime_ = None
         if datetime:
             datetime_ = dt_util.parse_datetime(datetime)
 

tests/components/history/test_init.py

@@ -768,6 +768,16 @@ async def test_fetch_period_api_with_minimal_response(hass, hass_client):
     assert response.status == 200
 
 
+async def test_fetch_period_api_with_no_timestamp(hass, hass_client):
+    """Test the fetch period view for history with no timestamp."""
+    await hass.async_add_executor_job(init_recorder_component, hass)
+    await async_setup_component(hass, "history", {})
+    await hass.async_add_job(hass.data[recorder.DATA_INSTANCE].block_till_done)
+    client = await hass_client()
+    response = await client.get("/api/history/period")
+    assert response.status == 200
+
+
 async def test_fetch_period_api_with_include_order(hass, hass_client):
     """Test the fetch period view for history."""
     await hass.async_add_executor_job(init_recorder_component, hass)