tracker issue : CF-4207281

Problem Description:
the wddx serialization process converts undefined values to null, which in turn get deserialized to an empty string, so undefined values do not stay undefined.

Steps to Reproduce:
function test(a, b, c) {
	writeDump(arguments); // shows initial structure 
	
	cfwddx(action="cfml2wddx", input=arguments, output="local.payload");
	writeDump(local.payload); // shows incorrect conversion of undefined argument c to empty string during serialization
	writeOutput(htmlCodeFormat(local.payload)); // shows undefined argument c serialized as <null/>
	
	cfwddx(action="wddx2cfml", input=local.payload, output="local.vars");
	writeDump(local.vars); // shows deserialized value of c is different from initial value
}
test(1, "hello");

Actual Result:
undefined values are serialized as <null /> in the wddx structure which is deserialized as an empty string

Expected Result:
undefined values should be omitted from the serialized result (this is what older versions of CF did, i.e. CF11)

Any Workarounds:
must check for empty string after checking for existence in any struct obtained from a wddx serialized source. -- No workaround if empty string is a valid value.

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