Use nil, not "" for unused lines in file metadata

This makes the DBs marginally larger (compared to using "" - they're still much
smaller than in 1.1.2), but reduces memory usage on re-reading a DB (when all
the "" are not just references to the same object).
Note the improved read/write time and mem usage

As carry-on effects from less duplicated storage of lines in the database.
Merge pull request #78 from eapache/null-missing-lines

Null missing lines