Oh, I see. It appears that there's a special case for partitioned tables that sets relpages=-1 in do_analyze_rel() around line 680. It's a bit inconsistent, though, because even partitioned indexes have relpages=0. Furthermore, the parameter is of type BlockNumber, so InvalidBlockNumber would make more sense.
Not the cleanest code, but if the value exists, we need to be able to import it.
Thanks for tracking that down. I'll have a patch ready shortly.