Problem: even if an Item_xml_str_func successor returns NULL, it doesn't have 
a corresponding property (maybe_null) set, that leads to a failed assertion.

Fix: set nullability property of Item_xml_str_func.


mysql-test/r/xml.result:
  Fix for bug #32557: order by updatexml causes assertion in filesort
    - test result.
mysql-test/t/xml.test:
  Fix for bug #32557: order by updatexml causes assertion in filesort
    - test case.
sql/item_xmlfunc.h:
  Fix for bug #32557: order by updatexml causes assertion in filesort
    - set Item_xml_str_func::maybe_null.