USB null modem cable using 2 x PL2303: are resistors necessary?
No they are not needed for it to work. TTL serial inputs are designed to be high impedance and do not need the lines to have a specific termination resistance or anything. Most TTL uarts are normally directly connected. 

They do help if the lines somehow get shorted out, limiting the current to V/100 = I amps. Assuming 5V, that's 0.05 Amps or 50 millions, much better than a dead short.