Added support for empty field arrays in mappings

Fix for #6133, added the ability to send empty arrays as part of an index mapping json. For single and multi field properties objects

Sorry for the import changes, IntelliJ doesn't like eclipse imports and fights it like the devil.

If the test are in a non standard format please advise. I just find them easier to read like this.


I think we should not just ignore when something else than a map is provided? Maybe we could do something like:

```java
} else if (propName.equals("fields") {
  final Map<String, Object> multiFieldsPropNodes;
  if (propNode instance of List && ((List<?>) propNode.isEmpty()) {
    multiFieldsPropNodes = Collections.emptyMap();
  } else if (propNode instanceof Map) {
    multiFieldsPropNodes = (Map<String, Object>) propNode;
  } else {
    throw new MapperParsingException("Expected map for property [fields] on field [" + multiFieldName + "] or [" + type + "] but got a " + propNode.getClass());
  }
}
```