refactor(defaults)!: change default 'commentstring' value to empty (#22862)

test/functional/api/buffer_updates_spec.lua

@@ -75,7 +75,7 @@ local function reopenwithfolds(b)
   local tick = reopen(b, origlines)
 
   -- use markers for folds, make all folds open by default
-  command('setlocal foldmethod=marker foldlevel=20')
+  command('setlocal foldmethod=marker foldlevel=20 commentstring=/*%s*/')
 
   -- add a fold
   command('2,4fold')

test/functional/legacy/window_cmd_spec.lua

@@ -116,6 +116,7 @@ describe('splitkeep', function()
   -- oldtest: Test_splitkeep_fold()
   it('does not scroll when window has closed folds', function()
     exec([[
+      set commentstring=/*%s*/
       set splitkeep=screen
       set foldmethod=marker
       set number

test/functional/ui/fold_spec.lua

@@ -2032,8 +2032,11 @@ describe("folded lines", function()
   end)
 
   it('multibyte fold markers work #20438', function()
-    meths.win_set_option(0, 'foldmethod', 'marker')
-    meths.win_set_option(0, 'foldmarker', '«,»')
+    exec([[
+      setlocal foldmethod=marker
+      setlocal foldmarker=«,»
+      setlocal commentstring=/*%s*/
+    ]])
     insert([[
       bbbbb
       bbbbb