Currently an empty string is used to unset the location. Once
again, it makes more sense to use a null value for this purpose
(though the special behavior of empty strings is retained).
The code comment above the function also explicitly indicates
that null should be accepted, and the function does return null
rather than an empty string for the old location value (if it
is missing).
$typeName, $typeNamespace, $nodeName and $nodeNamespace all
special-case the empty string and don't set the property entirely
in that case. It makes more sense to use null to indicate absence
here (though of course the empty string behavior is retained).
