Document with no term vectors mixed with ones that have term vectors cause EOFException during merge

Another spinoff from here:

http://www.gossamer-threads.com/lists/lucene/java-dev/53306

Thank you to Andi Vajda for capturing the issue in a compact test!

This is the same logical error from LUCENE-1008, but in this case the
bug is in TermVectorsWriter: we are failing to write the "0" field
count to the tvd file when the document has no vectors. I have a unit
test showing the issue & simple fix.