FIX Add protection against deprecated warning from mb_check_encoding()

When the `$fieldValue` is not set, the `mb_check_encoding()` function raises a deprecation warning.

```
[Deprecated] mb_check_encoding(): Calling mb_check_encoding() without argument is deprecated
```

So we make sure the field value is set before using it.

src/DataFormatter/XMLDataFormatter.php

@@ -126,7 +126,7 @@ public function convertDataObjectWithoutHeader(DataObject $obj, $fields = null,
                 continue;
             }
             $fieldValue = $obj->obj($fieldName)->forTemplate();
-            if (!mb_check_encoding($fieldValue, 'utf-8')) {
+            if (isset($fieldValue) && !mb_check_encoding($fieldValue, 'utf-8')) {
                 $fieldValue = "(data is badly encoded)";
             }