Made Postgresql.null an empty string physically unequal to any other empty string

Author: mmottl

src/postgresql.ml

@@ -238,7 +238,7 @@ let ftype_of_string = function
 
 external init : unit -> unit = "PQocaml_init"
 
-let null = ""
+let null = String.make 0 '\000'
 
 let () =
   Callback.register_exception "Postgresql.Oid" (Oid invalid_oid);

src/postgresql.mli

@@ -212,7 +212,8 @@ val invalid_oid : oid
 
 val null : string
 (** [null] can be used as an element of the optional argument [parameters]
-    passed to the [exec] or [send_query] method to indicate a NULL value. *)
+    passed to the [exec] or [send_query] method to indicate a NULL value. It is
+    an empty string, but not physically equal to [""]. *)
 
 (** Class type of query results.