If an otyugh's bite attack misses, does the target have to make a saving throw?

Let's consider the following example of an action that an enemy (Guardian Naga) can perform during its turn (emphasis mine):

Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one creature. Hit: 8 (1d8 + 4) piercing damage, and the target must make a DC 15 Constitution saving throw, taking 45 (10d8) poison damage on a failed save, or half as much damage on a successful one.

It's clear to me here that if the attack misses, then the target doesn't have to make the saving throw.

But then let's consider for example the following attack (Otyugh):

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 12 (2d8 + 3) piercing damage. If the target is a creature, it must succeed on a DC 15 Constitution saving throw against disease or become poisoned until the disease is cured. [...]

In this case, if the attack misses, does the target have to make the saving throw or not?

Your parsing isn't wrong, but not perfect

The text is defined like this, as you quoted. For instance, the guardian naga's Bite attack says:

Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one creature. Hit: 8 (1d8 + 4) piercing damage, and the target must make a DC 15 Constitution saving throw, taking 45 (10d8) poison damage on a failed save, or half as much damage on a successful one.

But you don't need to alter the sentences to understand it. Why? Because it consists of distinct clauses:

Bite

Melee Weapon Attack: +8 to hit, reach 10 ft., one creature.

Hit: 8 (1d8 + 4) piercing damage, and the target must make a DC 15 Constitution saving throw, taking 45 (10d8) poison damage on a failed save, or half as much damage on a successful one.

If anything, you just need to inject an "on" before the word "Hit", making everything in the sentence after it conditional on the Bite attack hitting.

If the second sentence was for any other trigger, it would be noted as such. This is done akin to the Miss: in the block of the Piercer from the Monster Manual (p. 252) and other monsters:

Hit: 3 (1d6); piercing damage per 10 feet fallen, up to 21 (6d6).

Miss: The piercer takes half the normal falling damage for the distance fallen.

Similarly, the Harengon Sniper (The Wild Beyond the Witchlight, p. 235) features both a hit and a hit-or-miss effect:

Hit: 7 (1d8 + 3) piercing damage.

Hit or Miss: Immediately after making this attack, the harengon can use the Hide action.

So the Otyugh's effect triggers on a hit that targeted a creature. If it hit a non-creature (like a structure, object, or item), that part is ignored.