SQL ISO Standard for group concatenation
The wiscorp.com page SQL Standards has several older revisions and drafts of SQL:20nn (zip):

The 7IWD2-02-Foundation-2011-12.pdf, with a date of 2011-12-21 has at page 289:


  If at least one of S1 and S2 is the null value, then the result of the &lt;concatenation&gt; is the null value.