xSysdeviceFindFirstFile() does not check for last packet or packet length=0

If no files exist on the device (not even "." and ".."), on a DIR_LIST request, xSysdeviceFindFirstFile() will not check for the RCX_PACKET_SEQ_LAST mask and it will not check for a packet length of 0. This will result in an answered packet with an empty string in szName.

xSysdeviceFindFirstFile() should behave like xSysdeviceFindNextFile() and check ulExt for RCX_PACKET_SEQ_LAST or for check if ulLen = 0.

Seen on netX90 where DIR_LIST uses raw flash areas for files.