In UART serial, do some devices not have an RX connection?
The ultrasonic sensor module uses that serial TX to send distance data out. The sensor apparently does not need to receive any serial data from outside world. So you need only TX of sensor --> RX of Pi connection. TX of Pi is not needed. 

Since the sensor works with 3.3V range. You can use any 3.3V GPIO pin of Pi to give the pulse for acquiring real time data.