set javadoc to builder (#3681)

tools/java/java-build/src/com/google/i18n/phonenumbers/BuildMetadataProtoFromXml.java

@@ -408,7 +408,7 @@ public class BuildMetadataProtoFromXml extends Command {
 
     static Builder builder() {
       return new AutoValue_BuildMetadataProtoFromXml_ClassWriter.Builder()
-          .setModifiers("");
+          .setJavadoc("").setModifiers("");
     }
 
     @AutoValue.Builder