Revision 700815

Fix for a potential null-pointer-introducing bug which came about as part of the DocIdSet changes. TermsFilter no longer implemented bits(IndexReader) and the Filter base class' version of this was changed to return null. 
When dropping 2.4 Lucene in as a direct replacement for 2.3.2 my client code was getting NullPointer errors - returning null was never part of the Filter.bits contract and so this could be a problem for others using this class. 

Fix is for TermsFilter to implement bits(IndexReader) - this can safely be removed in later versions because it is a deprecated method going forward.