Error if an enum field allows null and is invalid

Ajv requires that nullable enum fields include `null` in the enum array with a schema like this:
```js
{
type: 'object',
properties: {
test: {
type: 'string',
enum: ['a', 'b', null],
nullable: true
},
metadata: reviewMetadataSchema
},
}
```

If you pass in `{ test: 'c' }` then `ajv.errors` will look like this
```js
[
{
instancePath: '/test',
schemaPath: '#/properties/test/enum',
keyword: 'enum',
params: {
allowedValues: ['a', 'b', null]
},
message: 'must be equal to one of the allowed values'
}
]
```

and better-ajv-errors will throw an error like this
```
TypeError: Cannot read properties of null (reading 'length')
at leven (/node_modules/leven/index.js:14:11)
at /node_modules/better-ajv-errors/src/validation-errors/enum.js:63:17
at Array.map ()
at EnumValidationError.findBestMatch (/node_modules/better-ajv-errors/src/validation-errors/enum.js:61:8)
at EnumValidationError.getError (/node_modules/better-ajv-errors/src/validation-errors/enum.js:30:28)
at customErrorToStructure (/node_modules/better-ajv-errors/src/index.js:11:49)
at Array.map ()
at src_default (/node_modules/better-ajv-errors/src/index.js:22:25)
```

Without the `null` in the enum, better-ajv-errors will work as expected, but then you can't make the property nullable.