patch 8.2.4168: disallowing empty function name breaks existing plugins

Problem:    Disallowing empty function name breaks existing plugins.
Solution:   Allow empty function name in legacy script.
diff --git a/src/testdir/test_autoload.vim b/src/testdir/test_autoload.vim
index 384ee1a..21de655 100644
--- a/src/testdir/test_autoload.vim
+++ b/src/testdir/test_autoload.vim
@@ -10,6 +10,9 @@
   call assert_equal(1, g:called_foo_bar_echo)
 
   eval 'bar'->g:foo#addFoo()->assert_equal('barfoo')
+
+  " empty name works in legacy script
+  call assert_equal('empty', foo#())
 endfunc
 
 func Test_source_autoload()
@@ -23,4 +26,5 @@
   call assert_equal(49, auto9#add42(7))
 endfunc
 
+
 " vim: shiftwidth=2 sts=2 expandtab