Item::val_str() function should set null_value to false if the value is not
null, and true if the value is null. But Item_func_char::val_str() doesn't
set it. In this case, once a invalid code point is passed to CHAR(),
Item_func_char's null_value is set to true, and the stored procedure uses
this value for each code point in later loop / run and there is no chance to
set it to false even if the code point is valid.

Fix:
Make Item_func_char::val_str() set null_value as suspected.

Change-Id: I759eb3d50dec1befe89c791fd7c405614c2a9e7d