Device class attribute for sensor component (#1525)

* Add constants for sensor device_class

* Add device_class attribute to sensor component

* Add device_class attribute to sensor class

* Add device_class to mhz19 temperature sensor

* Add device_class to sensor in api component

* Add test for device_class of sensor

* Rename DEVICE_CLASS_NONE to DEVICE_CLASS_EMPTY for consistency

* Make optional attributes of sensor component truly optional

tests/component_tests/sensor/test_sensor.py

@@ -0,0 +1,14 @@
+""" Tests for the sensor component """
+
+
+def test_sensor_device_class_set(generate_main):
+    """
+    When the device_class of sensor is set in the yaml file, it should be registered in main
+    """
+    # Given
+
+    # When
+    main_cpp = generate_main("tests/component_tests/sensor/test_sensor.yaml")
+
+    # Then
+    assert "s_1->set_device_class(\"voltage\");" in main_cpp

tests/component_tests/sensor/test_sensor.yaml

@@ -0,0 +1,12 @@
+esphome:
+  name: test
+  platform: ESP8266
+  board: d1_mini_lite
+
+sensor:
+  - platform: adc
+    pin: A0
+    id: s_1
+    name: "test s1"
+    update_interval: 60s
+    device_class: "voltage"