Evaluate the expression, returning the result as an XdmValue (that is,
a sequence of nodes and/or atomic values).
An XdmValue representing the results of the expression.
Although a singleton result may be represented as an XdmItem, there is
no guarantee that this will always be the case. If you know that the expression will return at
most one node or atomic value, it is best to use the EvaluateSingle method, which
does guarantee that an XdmItem (or null) will be returned.