runtime(termdebug): Fix saved_mousemodel check

Fix issue introduced by 23f29ffc64276dd790581f98b86a0a6435b7eb22 where
saved_mousemodel is introduced as a variable, so the exists check will
always be true.

Move to check the value of saved_mousemodel instead.

closes: #14946

Signed-off-by: laburnumT <laburnumtec@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/pack/dist/opt/termdebug/plugin/termdebug.vim b/runtime/pack/dist/opt/termdebug/plugin/termdebug.vim
index d083b7e..26c2665 100644
--- a/runtime/pack/dist/opt/termdebug/plugin/termdebug.vim
+++ b/runtime/pack/dist/opt/termdebug/plugin/termdebug.vim
@@ -1220,7 +1220,7 @@
     win_gotoid(curwinid)
     winbar_winids = []
 
-    if exists('saved_mousemodel')
+    if saved_mousemodel != ''
       &mousemodel = saved_mousemodel
       saved_mousemodel = null_string
       try