@hapi/validate schemas in the context of hapi-swagger use

#### Support plan

* *is this issue currently blocking your project?* (yes/no): no
* *is this issue affecting a production system?* (yes/no): no

#### Context

* *node version*: 14.16.0
* *module version*: 20.1.0
* *environment* (e.g. node, browser, native): node
* *used with* (e.g. hapi application, another framework, standalone, ...): hapi-swagger@14.1.0
* *any other relevant information*:

#### How can we help?

I'm starting an issue here, but I don't really know if it's up to hapi or hapi-swagger to fix the issue, I'd say hapi, but you may disagree.
There is currently an error thrown from hapi-swagger that looks like this:
```
Error: Cannot mix different versions of joi schemas
at new module.exports (./node_modules/@hapi/hoek/lib/error.js:23:19)
at module.exports (./node_modules/@hapi/hoek/lib/assert.js:20:11)
at Object.exports.isSchema (./node_modules/joi/lib/common.js:132:5)
at Object.utilities.isJoi (./node_modules/hapi-swagger/lib/utilities.js:236:24)
at Object.utilities.hasJoiChildren (./node_modules/hapi-swagger/lib/utilities.js:246:19)
at ./node_modules/hapi-swagger/lib/paths.js:291:21
[...]
```

The root cause is coming from [this line](https://github.com/hapijs/hapi/blob/bae179241e756f45ffee85184782c06cb3cbc4fa/lib/validation.js#L27). When hapi-swagger is checking that all the schemas are joi objects, this one is not, so the throw is totally expected.

Now I see 2 solutions to that :
- use the validator to create that schema so that it uses the correct joi version, but it doesn't seem easy to add the `allow(null)` part if we're not sure that we're getting a joi object back from the validator
- keep the false as it is and do some special treatment during the validation, but it has imho one major drawback, it's going to be hard to throw joi-compatible errors, which could cause some problems for people catching errors in failAction or any other hook

What do you think?