position of null carriers in N=256 subcarier OFDM
Positioning of null carriers in an OFDM signal is a protocol-dependent piece of information. Specifications will often call for some null carriers around the edge of the band (to allow for rolloff in the antialiasing filters in the front end). Another common place you'll find a null carrier is at the band center. This may be found in specifications that are targeted at low-cost (i.e. zero-IF) receiver technology. Zero-IF receivers' baseband DC offsets can cause large spurious responses in the tuned band center, hence the use of a null carrier there.

So, the short answer to your question is that it's unanswerable: it's dependent on the specifics of the system you're working with.