Per the manual:
Going through the actions of cocking, aiming and pulling the trigger on an unloaded gun is known as “dry firing.” It can be useful to learn the “feel” of your pistol. Be certain that the pistol is fully unloaded (both the chamber and magazine are empty) and that the pistol is pointing in a safe direction even when you are practicing dry firing. The RUGER® SR-SERIES pistols can be dry fired without damage to the striker or other components as long as an empty magazine is inserted.
CAUTION: Dry firing your RUGER® SR-SERIES with the magazine removed may result in damage or unnecessary wear to the magazine disconnect mechanism and/or striker.
NOTE: When dry firing the SR-SERIES pistols, the trigger will not automatically move forward to reset. The slide must be retracted at least 1/2” to reset the trigger and firing mechanism.
So, without a mag inserted, the trigger can be pulled but the striker will hit the mag disconnect not travel forward enough to fire a round.