[Format] Clarify whether 0-length variable offsets buffers are permissible for 0-length arrays in the IPC protocol

This aspect of the columnar format / IPC protocol remains slightly unclear. As written, it would suggest that an offsets buffer of length 1 containing a single value 0 is required. It may be better to allow this to be length zero (corresponding to a 0-size or null buffer in the implementation)