patch 8.2.4332: Vim9: incomplete test for existing script variable in block

Problem:    Vim9: incomplete test for existing script variable in block.
Solution:   Add a couple more tests.  Fix uncovered problem.
diff --git a/src/vim9script.c b/src/vim9script.c
index 8721088..1b8988d 100644
--- a/src/vim9script.c
+++ b/src/vim9script.c
@@ -600,7 +600,8 @@
 	    goto erret;
 	}
 	else if (imported == NULL
-		&& check_defined(as_name, STRLEN(as_name), cctx, FALSE) == FAIL)
+		&& check_defined(as_name, STRLEN(as_name), cctx, NULL,
+								FALSE) == FAIL)
 	    goto erret;
 
 	if (imported == NULL)