How to pass a NULL value in a message to a queue in SQL Server

The SEND Transact-SQL verb does not allow to send a NULL message body, attempting to do so will result in error:

Msg 8433, Level 16, State 1, Line 11
The message body may not be NULL.  
A zero-length UNICODE or binary string is allowed.

But there are ways to send a NULL message body. One way is to completely omit the message body argument:

SEND ON CONVERSATION @handle MESSAGE TYPE [...];

Another way is to send a 0 length message body, which will be enqueued as a NULL message body in the target queue:

SEND ON CONVERSATION @handle MESSAGE TYPE [...] (0x);
SEND ON CONVERSATION @handle MESSAGE TYPE [...] ('');
SEND ON CONVERSATION @handle MESSAGE TYPE [...] (N'');

All three forms above will enqueue the same message body: NULL. This is true for both binary messages (VALIDATION = NONE) and for XML messages (VALIDATION=WELL_FORMED_XML).

Here is a short test script showing this:

create message type [BINARY] validation = none;
create message type [XML] validation = well_formed_xml;
go

create contract [TEST] (
	[BINARY] sent by initiator,
	[XML] sent by initiator);
go

create queue Sender;
create service Sender on queue Sender;
go

create queue Receiver;
create service Receiver on queue Receiver  ([TEST]);
go

declare @h uniqueidentifier;

begin dialog conversation @h
from service [Sender]
to service N'Receiver', N'current database'
on contract [TEST]
with encryption = off;

send on conversation @h message type [BINARY];
send on conversation @h message type [BINARY] (0x);

send on conversation @h message type [XML];
send on conversation @h message type [XML] ('');
send on conversation @h message type [XML] (N'');

receive * from [Receiver];
go

The received message_body column has a NULL value for all 5 messages sent.