Add a check for the watchdog table before trying to truncate

I usually have dblog module turned off. When you do this, the watchdog table isn't there. I noticed using drush it'll throw an exception saying:

Base table or view not found: 1146 Table [error]
'new.watchdog' doesn't exist: TRUNCATE {watchdog};

Maybe a check can be added for this service to see if the table exists before trying to truncate it.

Nice module BTW!