patch 8.2.1949: Vim9: using extend() on null dict is silently ignored

Problem:    Vim9: using extend() on null dict is silently ignored.
Solution:   Give an error message.  Initialize a dict variable with an empty
            dictionary. (closes #7251)
diff --git a/src/evalvars.c b/src/evalvars.c
index a5db6e9..9c05c57 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -2553,7 +2553,22 @@
 	    ret = FAIL;
 	}
 	else if (rettv != NULL)
+	{
+	    // If a list or dict variable wasn't initialized, do it now.
+	    if (tv->v_type == VAR_DICT && tv->vval.v_dict == NULL)
+	    {
+		tv->vval.v_dict = dict_alloc();
+		if (tv->vval.v_dict != NULL)
+		    ++tv->vval.v_dict->dv_refcount;
+	    }
+	    else if (tv->v_type == VAR_LIST && tv->vval.v_list == NULL)
+	    {
+		tv->vval.v_list = list_alloc();
+		if (tv->vval.v_list != NULL)
+		    ++tv->vval.v_list->lv_refcount;
+	    }
 	    copy_tv(tv, rettv);
+	}
     }
 
     name[len] = cc;