SEC16 — More nil/NULL arguments that need to be removed

Summary: As we realized during the RTF 1.5, we have erroneous text for
operations that describe passing nil/NULL for parameters. I"ve
tracked down a few more:

p. 15-96 [495]: For the requested_privileges parameter, the phrase "(A
null attribute set requests default attributes.)" should read "A
zero length attribute list requests the default attributes."

p. 15-130 [666]: For the data_included_in_token parameter, the phrase
"(may be null)" should read "(may be a zero length sequence)".

p. 15-156 [784]: The object_type (which is a repository id) parameter
is allowed to be nil. It should read "If this is the empty string,
all types are implied."

p. 15-157 [787]: Same as above.

p. 15-171 [846]: For the mechanism parameter, the phrase "Normally
NULL" should read "Normally the empty string".

p. 15-171 [846]: For the chain_bindings parameter, the phrase
"Normally NULL (zero length)" should read "Normally a zero length
octet sequence".

Close issue 2344: More nil/NULL argument that need to be removed: