patch 8.2.0493: Vim9: some error messages not tested

Problem:    Vim9: some error messages not tested.
Solution:   Add more tests.  Fix uncovered bugs.
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 74c3d1c..3c0a1ae 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -2861,6 +2861,7 @@
 test_null_blob()		Blob	null value for testing
 test_null_channel()		Channel	null value for testing
 test_null_dict()		Dict	null value for testing
+test_null_function()		Funcref	null value for testing
 test_null_job()			Job	null value for testing
 test_null_list()		List	null value for testing
 test_null_partial()		Funcref	null value for testing