I don't have any strong feelings about this line of code, but let me at least explain it.
I like the idea that IIFoo behaves the same way a SegementFoo, even during incorrect/undocumented use of the API.
There are no real use cases for this in the Lucene distribution, there are however effects people might use even though caused by invalid use of the API and not recommened. E.g. a skipTo to a target greater than the greatest document associated with that term will position the enum at the greatest document number for that term. Even though I wouldn't do something like this others might.
In this case, where an immediate #next() on IR#termDocs() is called, it's might look silly to compare the behaviour of II and Segment as it's such blatantly erroneous use of the API, but even I have been known to come up with some rather strange solution now and then when nobody else is looking.
One alternative is that #next would produce an InvalidStateException or something instead of just accepting the call, but then there is of course the small extra cost associated with checking if the enum has been seeked yet and #next is a rather commonly used method.