PubSub should allow a NULL to replace a list of NULLs

[SamCooper]

In 3.5.6.8 m & u it states that:

For each update being notified there shall be an entry in the UpdateHeader list and an
entry in the update value lists.

For the situation where one of the value lists contains all NULLs, it should be possible to replace the complete list with a single NULL to be space efficient.

Also note that 3.5.6.8 has two sub-numbered item lists both starting from (a) so in effect there are two 3.5.6.8.a numbered entries.