VS 2010 / Json Replace Null Values With Empty String?

If I have JSON like this and pass it to the engine I am using to create a PDF file, the word "null" appears on the PDF. This isn't really my example, but I googled "json replace null empty string" and this was a hit, which is a good representation of my data, also:

{
"relationship": {
"type": "relationship",
"id": null,
"followed_id": null
}
}

We use a rendering engine that creates a PDF from a fixed template and variable data in the form of a json string. But our customers don't want to read that the id is "null", they want to read that the id is blank. In my C# program, the json is just a string, so I can easily do something like this:

jsonString = jsonString.Replace("null,", ",");but the issue is I can't replace null-comma with a comma, because then it's invalid json.

how to I make the rendering engine *and* my customer both happy?