This GM would have a creature that makes a successful Will save against masochistic shadow be unaffected by the whole spell
The spell masochistic shadow has the entry Saving Throw: Will negates, then Reflex partial; see text. On Magic on Saving Throw on Negates says, "The spell has no effect on a subject that makes a successful saving throw."
While that see below in the spell's Saving Throw entry is potentially meaningful, this GM would err on the side of the spell's effect being negated by success on the initial Will saving throw. That is, that the definition of Negates in conjunction with the spell's description makes this GM believe that success on the initial saving throw prevents the entire spell from affecting the target.
That is, first the spell's description explains its initial effect:
You animate the target’s shadow with semi-living energies drawn from the Shadow Plane, instilling a maddening hunger for its owner’s life energy within it. If the target fails its Will save, it takes 1d4 points of Strength damage as a quasi-real shadow manifests in its space and attacks it. This shadow remains attached to the target and moves wherever the target moves.
A successful saving throw negates this effect. Then the spell's description continues, explaining its round-by-round effect:
At the start of each subsequent round, the target must succeed at a Reflex save or take 1d4 additional points of Strength damage; a successful save reduces the Strength damage to 1 point. If its Strength score is reduced to 0 by this spell’s effects, the target dies. If the target is in bright light, it gains a +2 bonus on Reflex saves against this spell. If the target is in darkness, it takes a –2 penalty on Reflex saves against this spell.
(Emphases mine.) That word additional is important. If the subject was dealt no damage initially because it succeeded on the saving throw, presumably the subject also can't be dealt additional damage.
Many spells have flowery descriptions that explain what effects occur if the spell has its full effect. For example, the lesser geas spell says, "A lesser geas places a magical command on a creature to carry out some service or to refrain from some action or course of activity, as desired by you," mentioning nothing about the subject needing to fail a Will saving throw against the spell for that effect to occur. Similarly, the lightning bolt spell says, "You release a powerful stroke of electrical energy that deals 1d6 points of electricity damage per caster level (maximum 10d6) to each creature within its area," yet mentions nothing about this damage being halved on a successful Reflex saving throw. This GM believes that, like those spells, the masochistic shadow spell's flowery description illustrating what occurs if the subject fails the initial saving throw.
Although this reader agrees that relying on a spell's Saving Throw entry to manage a spell instead of writing a more complete description can lead to some misleading spell descriptions, the alternative—each spell describing exactly what happens on both a failed and a successful saving throw—would likely see spell descriptions both double in length and make the same statements repeatedly, and this reader thinks that's pretty much exactly what a spell's Saving Throw entry is trying to avoid.