A rigorous mission that finds no life on Enceladus would still be a major astrobiology result, because it could show that habitability and actual biology are separate outcomes.
Cassini established the moon as plausibly habitable by detecting a global ocean, plume material linked to water-rock interaction, molecular hydrogen, diverse organics and Phosphorus at least 100 times Earth-ocean levels.
Those findings do not amount to life detection: hydrogen, methane and organics can arise abiotically, and plume samples are indirect because transport through fractures can dilute, sort or alter ocean material.
One fly-through with a null result would not be enough; a meaningful negative would require repeated sampling, multiple independent biosignatures, contamination controls and stated false-negative limits.
If such searches still came up empty, Enceladus could become a rare natural control for testing how often favorable chemistry actually turns into life.