patch 8.2.2340: win_execute() unexpectedly returns number zero when failing

Problem:    win_execute() unexpectedly returns number zero when failing.
Solution:   Return an empty string. (closes #7665)
diff --git a/src/testdir/test_execute_func.vim b/src/testdir/test_execute_func.vim
index e02b08e..0363717 100644
--- a/src/testdir/test_execute_func.vim
+++ b/src/testdir/test_execute_func.vim
@@ -89,6 +89,8 @@
   call win_gotoid(thiswin)
   let line = win_execute(otherwin, 'echo getline(1)')
   call assert_match('the new window', line)
+  let line = win_execute(134343, 'echo getline(1)')
+  call assert_equal('', line)
 
   if has('popupwin')
     let popupwin = popup_create('the popup win', {'line': 2, 'col': 3})