untar: treat the tag #\nul as a synonym for #\0

The GNU Tar manual (at
https://www.gnu.org/software/tar/manual/html_node/Standard.html) says
that older archives may have the typetag AREGTYPE (\0) instead of
REGTYPE ('0'), and that AREGTYPE should silently be treated as
REGTYPE.

racket/collects/file/untar.rkt

@@ -62,7 +62,7 @@
   (define checksum-bytes (read-bytes* 8 in))
   (define type-byte (integer->char (read-byte in)))
   (define type (case type-byte
-                 [(#\0) 'file]
+                 [(#\0 #\nul) 'file]
                  [(#\1) 'hard-link]
                  [(#\2) 'link]
                  [(#\3) 'character-special]