Return NULL from XML EXPLICIT subquery where no rows exist

Return NULL from XML EXPLICIT subquery where no rows exist

This is probably simple to do, but I'm having a brain-fart on this one...
I'm using FOR XML EXPLICIT as part of a subquery so that I can explicitly
define the format of the returned XML. Therefore I'm using UNION ALL to
define that format.
This is working fine, but I need it to return NULL if there are no rows in
that sub-query... at the moment it is returning an empty root element:
<codes/>. That is because I need the first row for the definition.
Here is a sqlfiddlecom with everything below for you to look at.
This is a version of the TSQL as it currently is...
SELECT
P.[PROJECTID],
P.[PROJECTNAME],
( SELECT *
FROM (
SELECT
1 AS TAG,
NULL AS PARENT,
NULL AS 'codes!1',
NULL AS 'code!2!!element',
NULL AS 'code!2!split'
UNION ALL
SELECT
2 AS TAG,
1 AS PARENT,
NULL,
C.[CODE],
C.[SPLIT]
FROM [CODES] C
WHERE C.[PROJECTID] = P.[PROJECTID]
) AS [CODEXMLDATA]
FOR XML EXPLICIT
) AS [CODESXML]
FROM [PROJECTS] P
Example data would be along the lines of
PROJECTS table
PROJECTID PROJECTNAME
1 This
2 That
3 Other
CODES table
PROJECTID CODE SPLIT
1 ABC 45
1 BCD 65
2 CDE 100
The result is coming out as...
PROJECTID PROJECTNAME CODESXML
1 This <codes><code split="45">ABC</code><code
split="55">BCD</code></codes>
2 That <codes><code split="100">CDE</code></codes>
3 Other <codes/>
The result I need is (note the NULL on the 3rd line)...
PROJECTID PROJECTNAME CODESXML
1 This <codes><code split="45">ABC</code><code
split="55">BCD</code></codes>
2 That <codes><code split="100">CDE</code></codes>
3 Other NULL
Can anybody give me a hint how I can make it return NULL when there are no
CODES?