seq, java: fix allocation bug in empty strings and test

Fixes golang/go#9271.

Change-Id: I57dcd11ae5afdc23bc9b1a0945d0789c9feeefb1
Reviewed-on: https://go-review.googlesource.com/1423
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>