vinyl: keep track of empty runs

We store some extra info in vylog per each run, even an empty one.
Currently, its min and max LSN over all statements written to the run
file so for an empty run they don't make sense right now. However, it's
going to change: soon we will store min and max LSN spanned by the run.
So we should keep track of empty runs. In order not to create empty data
files, we add is_empty flag to the vy_log_record, which indicates that
the run is empty and has no data file.