- Fixed: wrong FString empty string check.

Even when '+logfile' argument was omitted, the console would print 'Could not start log', because 'logfile != NULL' was used as a check for the presence of '+logfile' argument, but the internal buffer of FString is never NULL, so the right check is 'logfile.isNotEmpty()'.
While I'm at it, I fixed another bad check for 'pagename'.

It would be convenient if there could be a way to detect these mistakes in the code.