question about OCTET STRING encoding within an index

While testing and reviewing my SNMP test harness I've come
across an instance of a variable with an index I can't explain
(on my Fedora box).

snmpwalk -v1 -cpublic -On localhost vacmContextName
.1.3.6.1.6.3.16.1.1.1.1.0 = STRING:

So the index on this column is '.0' (which is defined as an OCTET STRING).

What I'm confused about is... is the zero:
a) the length of the non-existent NULL string that follows? or
b) is it a string that contains a NULL?

Normally I would have expected something like:
3.65.66.67 to represent the string 'ABC' where '3' is the length
(aka number of sub-identifiers)

variable length fields types as the 'n-1' parts of an index
MUST be length encoded, but ISTR that if its the last variable
in an index list, that the length always is omitted?  As in:

snmpwalk -v1 -cpublic -On localhost snmpNotifyRowStatus
.1.3.6.1.6.3.13.1.1.1.5.105.110.116.101.114.110.97.108.48 = INTEGER:
active(1)

where the index (a 'name' associated with the row) is
105.110.116.101.114.110.97.108.48  ie. 'internal0'
where there was no length specified.

Does this mean that an empty OCTET STRING (or OBJECT IDENTIFIER)
as the last INDEX component is a special case?

TIA
Fulko

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