patch 8.2.2159: Vim9: when declaring a list it is not allocated yet

Problem:    Vim9: when declaring a list it is not allocated yet, causing a
            following extend() to fail.
Solution:   When fetching a variable value for a list or dict that is null
            allocate the list or dict, so it can be used. (closes #7491)

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2159,
 /**/
     2158,
 /**/