The problem is that XML functions(items) do not reset null_value
before their execution and further item excution may use
null_value value of the previous result.
The fix is to reset null_value.


mysql-test/r/xml.result:
  test result
mysql-test/t/xml.test:
  test case
sql/item_xmlfunc.cc:
  The problem is that XML functions(items) do not reset null_value
  before their execution and further item excution may use
  null_value value of the previous result.
  The fix is to reset null_value.