XMLCONCAT takes a number of expressions as parameters. Each of these expressions results in a sequence. An item is equivalent to a sequence containing just that item. A NULL is treated like an empty sequence.
The result is a sequence containing the items in all the sequences in order.
The semantics of XML concatenation are specified in pairs where if either of the pair resolves to a NULL, the result is the other value. If both are NULL, the result is a NULL. Therefore in the event that all XML_value expr arguments are NULL, the result is a NULL.