fix setting root element and don't return empty values

Author: olafura

lib/microsoft_kiota_serialization_json/json_serialization_writer.rb

@@ -162,6 +162,7 @@ def write_object_value(key, value)
         if !key
           temp = JsonSerializationWriter.new()
           value.serialize(temp)
+          @writer = temp.writer
           return temp
         end
         begin
@@ -179,7 +180,7 @@ def write_enum_value(key, values)
     end
 
     def get_serialized_content()
-      return @writer.to_json #TODO encode to byte array to stay content type agnostic
+      return @writer.compact.to_json #TODO encode to byte array to stay content type agnostic
     end
 
     def write_additional_data(value)