SMHE-1602: improve CombinedDeviceModelCode and check for correct input

Signed-off-by: Andre Bolder <andre.bolder@alliander.com>

Author: Andre Bolder

osgp/protocol-adapter-dlms/osgp-protocol-adapter-dlms/src/test/java/org/opensmartgridplatform/adapter/protocol/dlms/domain/valueobjects/CombinedDeviceModelCodeTest.java

@@ -48,10 +48,10 @@ void parseShouldReturnEmptyCombinedDeviceModelCode() {
         CombinedDeviceModelCode.parse("invalid");
 
     Assertions.assertEquals("", combinedDeviceModelCode.getGatewayDeviceModelCode());
-    assertEquals(null, combinedDeviceModelCode.getCodeFromChannel(1));
-    assertEquals(null, combinedDeviceModelCode.getCodeFromChannel(2));
-    assertEquals(null, combinedDeviceModelCode.getCodeFromChannel(3));
-    assertEquals(null, combinedDeviceModelCode.getCodeFromChannel(4));
+    assertNull(combinedDeviceModelCode.getCodeFromChannel(1));
+    assertNull(combinedDeviceModelCode.getCodeFromChannel(2));
+    assertNull(combinedDeviceModelCode.getCodeFromChannel(3));
+    assertNull(combinedDeviceModelCode.getCodeFromChannel(4));
   }
 
   @Test