patch 8.2.3337: completing "call g:" returns entries with just "g:"

Problem:    Completing "call g:" returns entries with just "g:". (Naohiro Ono)
Solution:   Skip empty strings returned by get_user_func_name(). (closes #8753)
diff --git a/src/testdir/test_cmdline.vim b/src/testdir/test_cmdline.vim
index 90afe45..3c5513b 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -657,6 +657,11 @@
   " g: prefix also works
   call feedkeys(":echo g:Test_cmdline_complete_user_f\<Tab>\<Home>\"\<cr>", 'tx')
   call assert_match('"echo g:Test_cmdline_complete_user_func', @:)
+
+  " using g: prefix does not result in just "g:" matches from a lambda
+  let Fx = { a ->  a }
+  call feedkeys(":echo g:\<Tab>\<Home>\"\<cr>", 'tx')
+  call assert_match('"echo g:[A-Z]', @:)
 endfunc
 
 func Test_cmdline_complete_user_names()