patch 8.2.0493: Vim9: some error messages not tested

Problem:    Vim9: some error messages not tested.
Solution:   Add more tests.  Fix uncovered bugs.

runtime/doc/eval.txt

@@ -2861,6 +2861,7 @@ test_ignore_error({expr})	none	ignore a specific error
 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

runtime/doc/testing.txt

@@ -106,6 +106,10 @@ test_null_dict()					*test_null_dict()*
 		Return a |Dict| that is null. Only useful for testing.
 
 
+test_null_function()					*test_null_function()*
+		Return a |FuncRef| that is null. Only useful for testing.
+
+
 test_null_job()						*test_null_job()*
 		Return a |Job| that is null. Only useful for testing.
 		{only available when compiled with the +job feature}