This depends on a number of factors.
In theaters where movies are projected via 35mm film using a platter system (the entire movie is spliced together into a single “reel”), there is a chance that theater management may decide not to run the movie if nobody shows up. Once a film is started, there’s no way to stop and rewind, and “fast-forwarding” is a delicate procedure that ought to be avoided unless the projectionist knows what he or she is doing. It’s still a judgment call, though.
If it’s in the middle of the day, and the theater has other movies running, it will probably be started anyway on the off chance that someone may show up late. If it’s the last show of the day and it’s a long movie, there’s a better chance that it will not be run. Any time a theater has staff on the clock and no customers walking through the doors, it’s burning money, so a chance to shut down and send people home early is a chance to keep payroll costs under control.
With digital projectors, the film can be stopped at any point with no risk of damage to the print. Again, if it’s in the middle of the day, there’s probably no strong argument for choosing not to run the feature, but if it’s the last show of the night, and stopping it could get employees off the clock faster, it’s possible that the film could be stopped once it’s clear nobody is going to buy a ticket.