fix(api): reconfiguring float "relative" does not clear "win" (#34271)

Problem:  Unable to change the "relative" of a flag after its target
          "win" no longer exists.
Solution: Unset target window if it is not present in config and
          reconfigured "relative" no longer expects a target window.
(cherry picked from commit 963308439a)

src/nvim/api/win_config.c

@@ -1193,6 +1193,8 @@ static bool parse_win_config(win_T *wp, Dict(win_config) *config, WinConfig *fco
                     "non-float with 'win' requires at least 'split' or 'vertical'");
       goto fail;
     }
+  } else {
+    fconfig->window = 0;
   }
 
   if (HAS_KEY_X(config, external)) {

test/functional/ui/float_spec.lua

@@ -378,6 +378,21 @@ describe('float window', function()
     )
   end)
 
+  it('no error message when reconfig relative field on closed win', function()
+    command('split')
+    local winid = api.nvim_open_win(0, false, {
+      relative = 'win',
+      width = 1,
+      height = 1,
+      col = 1,
+      row = 1,
+    })
+    eq(1001, api.nvim_win_get_config(winid).win)
+    command('close')
+    api.nvim_win_set_config(winid, { relative = 'editor', row = 1, col = 1 })
+    eq(nil, api.nvim_win_get_config(winid).win)
+  end)
+
   it('is not operated on by windo when non-focusable #15374', function()
     command([[
       let winids = []