patch 8.2.4602: Vim9: not enough test coverage for executing :def function

Problem:    Vim9: not enough test coverage for executing :def function.
Solution:   Add a few more tests.  Fix uncovered problem.  Remove dead code.
diff --git a/src/vim9.h b/src/vim9.h
index 5fd8526..ed5f676 100644
--- a/src/vim9.h
+++ b/src/vim9.h
@@ -87,8 +87,8 @@
     ISN_PUSHS,		// push string isn_arg.string
     ISN_PUSHBLOB,	// push blob isn_arg.blob
     ISN_PUSHFUNC,	// push func isn_arg.string
-    ISN_PUSHCHANNEL,	// push channel isn_arg.channel
-    ISN_PUSHJOB,	// push channel isn_arg.job
+    ISN_PUSHCHANNEL,	// push NULL channel
+    ISN_PUSHJOB,	// push NULL job
     ISN_NEWLIST,	// push list from stack items, size is isn_arg.number
     ISN_NEWDICT,	// push dict from stack items, size is isn_arg.number
     ISN_NEWPARTIAL,	// push NULL partial