patch 8.2.4526: Vim9: cannot set variables to a null value

Problem:    Vim9: cannot set variables to a null value.
Solution:   Add null_list, null_job, etc.
diff --git a/src/vim9.h b/src/vim9.h
index 07dd639..a4fcc9b 100644
--- a/src/vim9.h
+++ b/src/vim9.h
@@ -91,6 +91,7 @@
     ISN_PUSHJOB,	// push channel isn_arg.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
 
     ISN_AUTOLOAD,	// get item from autoload import, function or variable