Empty Values in a JavaScript JSON Object

There will be some javascript on every page of my site that gets a JSON object embedded into a <script/> tag in my html. Sometimes the JSON object will look like this:

var pageData = {
		"navigation" : {
			"pageName" : "anypage-123", 
			"pageSite" : "anysite.com",
			"pageType" : "any",
			"sale-price" : 27.50
                        "price" : 55.00
		}
	};

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other times there will be no "sale-price". Is it better to leave the "sale-price" out altogether or can I do something like this:

var pageData = {
		"navigation" : {
			"pageName" : "anypage-123", 
			"pageSite" : "anysite.com",
			"pageType" : "any",
			"sale-price" : null,
                        "price" : "55.00"
		}
	};

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What's the best practice? Using the keyword "null" or leaving it blank and just have the comma?

Thanks in advance!
Aaron