fix(window): handle closing the only non-float in other tabpage

Problem: No check for closing the only non-floating window in a non-current
tabpage that contains floats. This can lead to a tabpage that contains only
floats, causing crashes.

Solution: Copy the relevant check from win_close to win_close_othertab. Fix some
uncovered issues.

Closes #34943
Fixes #31236

Co-authored-by: glepnir <glephunter@gmail.com>

test/functional/editor/tabpage_spec.lua

@@ -142,4 +142,13 @@ describe('tabpage', function()
     command('tabs')
     assert_alive()
   end)
+
+  it('no crash if autocmd remains in tabpage of closing last window', function()
+    exec([[
+      tabnew
+      let s:win = win_getid()
+      autocmd TabLeave * ++once tablast | tabonly
+      quit
+    ]])
+  end)
 end)