Sorry for the last commit, this is the better solution. As the offset attribute was normally already added before in the indexing process, the addAttribute is at no cost. And the additional if-clause is prevented (I missed a second one in the previous commit). Now only payloads are conditional to attribute existence.
lucene/java/trunk/src/java/org/apache/lucene/index/TermVectorsTermsWriterPerField.java