mapfile v2 syntax appears to preclude empty segments
As best as I can determine, the mapfile v2 language has no strict equivalent to the old "?E" flag, to state that a segment is (intentionally) empty.
You can work around this (have an object, put data in it, use the object path as the entrance criteria for your segment, for eg), but that's much more cumbersome than the old way. We should consider adding an "EMPTY" flag for at least LOAD_SEGMENT, that set FLG_SG_EMPTY, as the old ?E did.
I'm still in the process of, slowly, trying to determine whether the absence of such a thing is in fact a feature with sensible reasons behind it.