[entity] Allow `device_id` to be blank on entities (#10217)

tests/integration/fixtures/areas_and_devices.yaml

@@ -55,6 +55,12 @@ sensor:
     lambda: return 4.0;
     update_interval: 0.1s
 
+  - platform: template
+    name: Living Room Sensor
+    device_id: ""
+    lambda: return 5.0;
+    update_interval: 0.1s
+
 # Switches with the same name on different devices to test device_id lookup
 switch:
   # Switch with no device_id (defaults to 0)
@@ -96,3 +102,23 @@ switch:
       - logger.log: "Turning on Test Switch on Motion Detector"
     turn_off_action:
       - logger.log: "Turning off Test Switch on Motion Detector"
+
+  - platform: template
+    name: Living Room Blank Switch
+    device_id: ""
+    id: test_switch_blank_living_room
+    optimistic: true
+    turn_on_action:
+      - logger.log: "Turning on Living Room Blank Switch"
+    turn_off_action:
+      - logger.log: "Turning off Living Room Blank Switch"
+
+  - platform: template
+    name: Living Room None Switch
+    device_id:
+    id: test_switch_none_living_room
+    optimistic: true
+    turn_on_action:
+      - logger.log: "Turning on Living Room None Switch"
+    turn_off_action:
+      - logger.log: "Turning off Living Room None Switch"

tests/integration/test_areas_and_devices.py

@@ -132,6 +132,7 @@ async def test_areas_and_devices(
             "Temperature Sensor Reading": temp_sensor.device_id,
             "Motion Detector Status": motion_detector.device_id,
             "Smart Switch Power": smart_switch.device_id,
+            "Living Room Sensor": 0,  # Main device
         }
 
         for entity in sensor_entities:
@@ -160,6 +161,18 @@ async def test_areas_and_devices(
             "Should have a switch with device_id 0 (main device)"
         )
 
+        # Verify extra switches with blank and none device_id are correctly available
+        extra_switches = [
+            e for e in switch_entities if e.name.startswith("Living Room")
+        ]
+        assert len(extra_switches) == 2, (
+            f"Expected 2 extra switches for Living Room, got {len(extra_switches)}"
+        )
+        extra_switch_device_ids = [e.device_id for e in extra_switches]
+        assert all(d == 0 for d in extra_switch_device_ids), (
+            "All extra switches should have device_id 0 (main device)"
+        )
+
         # Wait for initial states to be received for all switches
         await asyncio.wait_for(initial_states_future, timeout=2.0)
 

tests/unit_tests/core/test_entity_helpers.py

@@ -689,3 +689,19 @@ def test_entity_duplicate_validator_internal_entities() -> None:
         Invalid, match=r"Duplicate sensor entity with name 'Temperature' found"
     ):
         validator(config4)
+
+
+def test_empty_or_null_device_id_on_entity() -> None:
+    """Test that empty or null device IDs are handled correctly."""
+    # Create validator for sensor platform
+    validator = entity_duplicate_validator("sensor")
+
+    # Entity with empty device_id should pass
+    config1 = {CONF_NAME: "Battery", CONF_DEVICE_ID: ""}
+    validated1 = validator(config1)
+    assert validated1 == config1
+
+    # Entity with None device_id should pass
+    config2 = {CONF_NAME: "Temperature", CONF_DEVICE_ID: None}
+    validated2 = validator(config2)
+    assert validated2 == config2