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/vim9compile.c b/src/vim9compile.c
index 06b6c79..fde8188 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -2238,10 +2238,10 @@
 			generate_NEWDICT(cctx, 0);
 			break;
 		    case VAR_JOB:
-			generate_PUSHJOB(cctx, NULL);
+			generate_PUSHJOB(cctx);
 			break;
 		    case VAR_CHANNEL:
-			generate_PUSHCHANNEL(cctx, NULL);
+			generate_PUSHCHANNEL(cctx);
 			break;
 		    case VAR_NUMBER:
 		    case VAR_UNKNOWN: