improve optionals guide

[EltonLobo07]

Improvements:

1. Replace Input and Output with InferInput and InferOutput in one of the examples.
2. Change the behavior of the Dependent default values example to enable TypeScript to infer the correct output type.

More information related to the 2nd improvement:
The example without the improvement does not work as intended. This might be because TS does not narrow the type of a generic function parameter (related) but using the ternary operator narrows the type and everything works as intended. There is one major difference between the current and improved examples. The current example returned the same object reference but the improved example doesn't when sum is not undefined and I think it is fine because:

1. The transform action is meant to transform values, so relying on referential equality should be discouraged.
2. I wasn't able to find a better way to let TS infer the type of the output (without a type assertion or something similar).

[pkg-pr-new]

Open in Stackblitz

npm i https://pkg.pr.new/valibot@948

commit: bbdbc90

[fabian-hiller]

Thank you! I chose the current implementation to not clone the object unnecessarily for performance reasons when not needed. But your code is ok too. But let us simplify the code and remove the return statement by writing () => ({ ... }).