For empty logfiles merely print a message and skip. Previously empty logfiles would require an explicit log format because otherwise detection would fail with the following message: "Fatal error: Cannot guess the logs format. Please give one using either the --log-format-name or --log-format-regex option"
See also: http://dev.piwik.org/trac/ticket/3163#comment:81
Might need to verify that this does not break using stdin as input file.
This is now in trunk