Make sql-null a transparent struct.

This allows it to cooperate better with Typed Racket, particularly
regarding the `Any` type. The guard and use of `#:authentic` also
check that it's still a singleton in all cases.

racket/collects/db/private/generic/sql-data.rkt

@@ -11,8 +11,16 @@
 ;; NULL
 
 (define-values (sql-null sql-null?)
-  (let ()
+  (let ([created? #false])
     (struct sql-null ()
+      #:transparent
+      #:authentic
+      #:guard (lambda (n)
+                (when created?
+                  (error 'sql-null "cannot create new instances of sql-null"))
+                (set! created? #true)
+                (values))
+      #:property prop:custom-write (lambda (v p w?) (write-string "#<sql-null>" p))
       ;; must deserialize to singleton, so can't just use serializable-struct
       #:property prop:serializable
       (make-serialize-info (lambda _ '#())