not always correct for NULL values.
Now they always result a non NULL value even
the argument is NULL. It is more usefull for 
debugging purposes.


mysql-test/r/func_str.result:
  Bugs#9129: CHARSET(), COLLATION(), COERCIBILITY()
  not always correct for NULL values.
  
  Also, a mistake in EXPORT_SET was fixed.
mysql-test/t/func_str.test:
  Bugs#9129: CHARSET(), COLLATION(), COERCIBILITY()
  not always correct for NULL values.
sql/item_func.cc:
  Bugs#9129: CHARSET(), COLLATION(), COERCIBILITY()
  not always correct for NULL values.
sql/item_strfunc.cc:
  Bugs#9129: CHARSET(), COLLATION(), COERCIBILITY()
  not always correct for NULL values.
  
  A minor mistake in EXPORT_SET() was fixed.