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_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index a84b7f6..40cfdcb 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -800,6 +800,11 @@
   timer_stop(id)
 enddef
 
+def Test_win_execute()
+  assert_equal("\n" .. winnr(), win_execute(win_getid(), 'echo winnr()'))
+  assert_equal('', win_execute(342343, 'echo winnr()'))
+enddef
+
 def Test_win_splitmove()
   split
   win_splitmove(1, 2, {vertical: true, rightbelow: true})