Fix compile error

Author: jordan-powers

x-pack/plugin/wildcard/src/main/java/org/elasticsearch/xpack/wildcard/mapper/WildcardFieldMapper.java

@@ -1104,7 +1104,12 @@ public FieldMapper.Builder getMergeBuilder() {
     protected SyntheticSourceSupport syntheticSourceSupport() {
         return new SyntheticSourceSupport.Native(() -> {
             var layers = new ArrayList<CompositeSyntheticFieldLoader.Layer>();
-            layers.add(new BinaryDocValuesSyntheticFieldLoaderLayer(fullPath()));
+            layers.add(new BinaryDocValuesSyntheticFieldLoaderLayer(fullPath()) {
+                @Override
+                protected void writeValue(XContentBuilder b, BytesRef value) throws IOException {
+                    b.utf8Value(value.bytes, value.offset, value.length);
+                }
+            });
             if (ignoreAbove.valuesPotentiallyIgnored()) {
                 layers.add(new CompositeSyntheticFieldLoader.StoredFieldLayer(originalName()) {
                     @Override