We should not allow `optional`, `nullish`, or `nullable` on `z.coerce` schemas

[scotttrinh]

Since z.coerce.* schemas will always return the primitive, even if you pass undefined or null, we should disallow this. Also, things like z.union([z.coerce.string(), z.coerce.number()]) will always return a string since the first schema will just always succeed.

[colinhacks]

Those methods wrap the existing schema with ZodOptional, ZodNullable, or both. So optionality still worked as intended, and the coercion behavior doesn't "bubble up"

z.coerce.string().optional().parse(undefined)
// => undefined

It's true that a coerced schema will never throw, so .catch() is a no-op. But tbh I don't much care about no ops. We could override these methods on certain subclasses to throw an error, or mark them with strikethrough using @deprecated. But I think that would introduce more confusion than it resolves.