One option is to perform some post-processing of the output using regular expressions:
First define the following Regex to find the empty tags:
static Regex emptyElementRegex = new Regex(@"<(\w+)\s*/>");
Then process your output to replace all matching occurrences:
var result = builder.ToString();
result = emptyElementRegex.Replace(result, @"<$1>");
This should transform ALL self-closing tags without attributes.