[Proposal] Support empty enum keyword with validateSchema: false option

**What version of Ajv are you using? Does the issue happen if you use the latest version?**
6.12.3 - no errors thrown
8.9.0 - throws an error
**Ajv options object**

```javascript
{validateSchema: false}
```

**JSON Schema**

```json
{enum: []}
```

**Sample data**

```json

```

**Your code**

```javascript
const ajv = new Ajv({validateSchema: false});
ajv.compile({enum:[]}) // throws
```

**Validation result, data AFTER validation, error messages**

```
Error is thrown: "enum must have a non-empty array"
```

**What results did you expect?**
No errors thrown during compilation if `validateSchema` is set to `false`
**Are you going to resolve the issue?**
I would give it a go with guidance on how the error should be handled.

Workaround for consumers of course is just to catch the error, but it's not something I would have expected and I couldn't see it listed as one of the intentional breaking changes.