[mqtt] Treat incoming empty string as NULL for enum (openhab#16641)

Treat incoming empty string as NULL for enum (openhab#16641)

Signed-off-by: Cody Cutrer <[email protected]>

Author: ccutrer

bundles/org.openhab.binding.mqtt.generic/src/main/java/org/openhab/binding/mqtt/generic/values/TextValue.java

@@ -106,7 +106,11 @@ public State parseMessage(Command command) throws IllegalArgumentException {
         final Set<String> states = this.states;
         String valueStr = command.toString();
         if (states != null && !states.contains(valueStr)) {
-            throw new IllegalArgumentException("Value " + valueStr + " not within range");
+            if (valueStr.isEmpty()) {
+                return UnDefType.NULL;
+            } else {
+                throw new IllegalArgumentException("Value " + valueStr + " not within range");
+            }
         }
         return new StringType(valueStr);
     }