patch 8.2.2033: Vim9: :def without argument gives compilation error

Problem:    Vim9: :def without argument gives compilation error.
Solution:   Add the DEF instruction. (closes #7344)

src/ex_docmd.c

@@ -274,7 +274,6 @@ static void	ex_tag_cmd(exarg_T *eap, char_u *name);
 # define ex_continue		ex_ni
 # define ex_debug		ex_ni
 # define ex_debuggreedy		ex_ni
-# define ex_def			ex_ni
 # define ex_defcompile		ex_ni
 # define ex_delfunction		ex_ni
 # define ex_disassemble		ex_ni