patch 9.1.0802: tests: no error check when setting global 'fdm' to empty value

Problem:  tests: no error check when setting global 'fdm' to empty value
Solution: Also check global 'fdm' value for being empty (Milly).

closes: #15916

Signed-off-by: Milly <milly.ca@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

Author: Milly

src/optionstr.c

@@ -2336,8 +2336,7 @@ did_set_foldmethod(optset_T *args)
 {
     char_u	**varp = (char_u **)args->os_varp;
 
-    if (check_opt_strings(*varp, p_fdm_values, FALSE) != OK
-	    || *curwin->w_p_fdm == NUL)
+    if (check_opt_strings(*varp, p_fdm_values, FALSE) != OK || **varp == NUL)
 	return e_invalid_argument;
 
     foldUpdateAll(curwin);

src/testdir/gen_opt_test.vim

@@ -49,7 +49,6 @@ let skip_setglobal_reasons = #{
       \ colorcolumn:	'TODO: fix missing error handling for setglobal',
       \ conceallevel:	'TODO: fix missing error handling for setglobal',
       \ foldcolumn:	'TODO: fix missing error handling for setglobal',
-      \ foldmethod:	'TODO: fix `setglobal fdm=` not given an error',
       \ iskeyword:	'TODO: fix missing error handling for setglobal',
       \ numberwidth:	'TODO: fix missing error handling for setglobal',
       \ scrolloff:	'TODO: fix missing error handling for setglobal',

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    802,
 /**/
     801,
 /**/