RF generator TX antenna with low impedance can damage generator?
You need not be worried.

Commercial generators like that are designed to be run on the bench. That means it is assumed they will be left running, at max power output, with no output cable connected (open circuit reflects just as much power as short circuit!). Under those circumstances, 100% of the output power is reflected back into the generator. They just would not survive in a lab if they were less robust than that. Your antenna will not reflect more than 100% of the incident power.

That generator has a maximum output power of +13dBm. That almost certainly means that when programmed to 0dBm output level, it will switch an output attenuator in there, and be running its output amplifier at a higher power. The attenuator will absorb the bulk of the reflected power.

If you read the data sheet, it says it is protected against reverse power applied to the output of up to 1 watt, +30dBm. The reason for that is sometimes, idiots in the lab, when testing the receive side of a radio transceiver, would key up the transmitter. Ooops! Again, all lab generators have an element of protection like this so they last beyond the first week of use.