This is a strange one, and I don't have anything to back it up, just one occurrence.
I attempted a CONCAT of a few fields, some of them being NULL values. If a row contained a field with a NULL value, the resulting concatenated string was also null.
This leads me to conclude that in the language of MySQL,
NULL + string = NULL
So, if you plan on using CONCAT on a particular set of fields, perhaps make them contain empty string values, as opposed to NULL.