backends/brass/: Only create a btree entry for the document data if
it is non-empty.  The table is now lazily created, so won't exist if
no documents have document data set.

git-svn-id: file:///home/xapian-svn/xapian/trunk@18221 6b2f1b92-a4f3-0310-943f-de3cbe41748b