add a nil catch to timestamp file sink

an additional nil guard is needed here just to be sure

lib/rex/logging/sinks/timestamp_flatfile.rb

@@ -12,6 +12,7 @@ module Sinks
 class TimestampFlatfile < Flatfile
 
   def log(sev, src, level, msg, from) # :nodoc:
+    return unless msg.present?
     msg = msg.chop.gsub(/\x1b\[[0-9;]*[mG]/,'').gsub(/[\x01-\x02]/, " ")
     fd.write("[#{get_current_timestamp}] #{msg}\n")
     fd.flush