[#3163] Update misc/log-analytics/import_logs.py to handle empty log files #11

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

Thanks!