I needed a feature such as in this StackOverflow question.

When you have a schema such as:

const schema = Joi.object({
  a: Joi.string().default(() => null),
  b: Joi.string().default(() => null)
}).oxor('a', 'b');

You may want schema.validate({}) to pass, however, currently this will fail because the oxor rule will resolve a and b to null, and the current check is against undefined.
This PR adds a new option, isPresent, that allows for something like:

const schema = Joi.object({
  a: Joi.string().default(() => null),
  b: Joi.string().default(() => null)
}).oxor('a', 'b', { isPresent: (resolved) => resolved !== undefined && resolved !== null });

In this case, the oxor (or any of the other dependency options, such as xor or and) will accept undefined and null values as empty.