Fix: "mysql --xml" now print NULL values the same way that "mysqldump --xml" does:
  
    <field name="name" xsi:nil="true" />
  
  to distinguish from empty strings:
  
    <field name="name"></field>
  
  and from string "NULL":
  
    <field name="name">NULL</field>


client/mysql.cc:
  Fixing to print NULLs differently from empty strings
mysql-test/r/client_xml.result:
  Fixing test result accordingly