Do not return error if there is no DB header.
Do not return error if there is no DB header.

- If a file is created & closed without any mutation, we cannot open
the file anymore.

Change-Id: I24615fac822025298c841901f31cb68790b65f97

couchbase/forestdb,hisundar/forestdb,hisundar/forestdb,couchbase/forestdb,wurikiji/forestdb,wurikiji/forestdb