How does the Void-Touched Sorcerer bloodline arcana work?

Question

The Bloodline Arcana for a Void-Touched Sorcerer reads as follows:

Whenever you cast an evocation spell, you may select one affected target that fails its save to suffer the choking airlessness of the void, silencing it (as silence, but affecting only the target) for 1 round. This is a supernatural ability.

This ability replaces the Starsoul Bloodline Arcana.

If this arcana is used with magic missile (an evocation spell), which has no saving throw, does the silence feature do anything?

The current assumption is that the evocation spell being used must have a saving throw in order for any possible failure to occur, to trigger silence.

Answer 1

The assumption's correct... and it's even worse than that

For the sorcerer's void-touched bloodline arcana to take effect, the sorcerer must cast a spell from the evocation school both that has as its target one or more creatures and that mandates making a saving throw. (A generous GM may waive the first restriction so that such a sorcerer has a chance of silencing one creature affected by—despite not being the target of—, for example, the spell fireball as the number of higher-level spells of the evocation school that target foes is very small.)

Thus, for example, the spells ear-piercing scream and touch of combustion combine with the void-touched bloodline arcana, but, for example, the spells burning hands and flare burst don't combine with the bloodline arcana as they don't have targets, and the spells magic missile, shocking grasp and thunderstomp don't combine with the bloodline arcana as they don't mandate saving throws

Answer 2

There was no save, so nobody failed it.

Thus, nobody qualifies to be selected and the arcana, just as you postulated, does nothing.

That still holds true if you use an Evocation spell with a save, but nobody fails it.