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/vim9execute.c b/src/vim9execute.c
index 56220f8..9f6cc5d 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -3440,6 +3440,17 @@
 		}
 		break;
 
+	    // create a partial with NULL value
+	    case ISN_NEWPARTIAL:
+		if (GA_GROW_FAILS(&ectx->ec_stack, 1))
+		    goto theend;
+		++ectx->ec_stack.ga_len;
+		tv = STACK_TV_BOT(-1);
+		tv->v_type = VAR_PARTIAL;
+		tv->v_lock = 0;
+		tv->vval.v_partial = NULL;
+		break;
+
 	    // call a :def function
 	    case ISN_DCALL:
 		SOURCING_LNUM = iptr->isn_lnum;
@@ -5720,6 +5731,9 @@
 		smsg("%s%4d NEWDICT size %lld", pfx, current,
 					    (varnumber_T)(iptr->isn_arg.number));
 		break;
+	    case ISN_NEWPARTIAL:
+		smsg("%s%4d NEWPARTIAL", pfx, current);
+		break;
 
 	    // function call
 	    case ISN_BCALL: