WL#1054: fixed a compilation warning

sql/sql_lex.cc

@@ -41,7 +41,7 @@ sys_var *trg_new_row_fake_var= (sys_var*) 0x01;
   LEX_STRING constant for null-string to be used in parser and other places.
 */
 const LEX_STRING null_lex_str= {NULL, 0};
-const LEX_STRING empty_lex_str= {"", 0};
+const LEX_STRING empty_lex_str= {(char *) "", 0};
 /**
   @note The order of the elements of this array must correspond to
   the order of elements in enum_binlog_stmt_unsafe.