If we have a field schema set to optional and the field value is empty, then the custom() validation is not triggered

### Runtime

node

### Runtime version

v22.14.0

### Module version

17.13.3

### Last module version without issue

_No response_

### Used with

react

### Any other relevant information

The issue is that if we have a field schema set to optional and the field value is empty, then the custom() validation is not triggered.
At the surface this seems trivial that a field with optional schema, having no value may not require validation, but that is not the case as the original issue creator mentions.

I am writing a wrapper to bring Yup, Zod and Joi under one standard library and it seems like Joi is the only one which has this problem. This is a deal breaker for me and if this issue is not fixed then I would abandon my Joi port.

Even google know this problem and gave me the following description of the problem.

It appears you're encountering an issue with Joi where a custom validation function isn't being executed when a field is marked as optional. Here's a breakdown of the problem and potential solutions:
Understanding the Issue:
Optional Fields: In Joi, fields are optional by default, meaning a value doesn't have to be present for validation to pass.
Custom Validation Logic: You might use .custom() or similar methods to implement custom validation logic for a field.
Problem: When a field is optional and not present, Joi might skip directly to passing validation, bypassing your custom logic.

Can this be "fixed"?

### What are you trying to achieve or the steps to reproduce?

Here's a breakdown of the problem and potential solutions:
Understanding the Issue:
Optional Fields: In Joi, fields are optional by default, meaning a value doesn't have to be present for validation to pass.
Custom Validation Logic: You might use .custom() or similar methods to implement custom validation logic for a field.
Problem: When a field is optional and not present, Joi might skip directly to passing validation, bypassing your custom logic.

### What was the result you got?

custom() validation not triggered when field value were empty

### What result did you expect?

custom() validation should be triggered regardless of the field value is empty or not.