patch 8.2.2291: Vim9: cannot use "null" for v:null

Problem:    Vim9: cannot use "null" for v:null.
Solution:   Support "null" like "true" and "false". (closes #7495)

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2291,
 /**/
     2290,
 /**/