The "better-ajv-errors" package fails to report a schema error

Nobody has claimed this yet.

When linting https://github.com/mdn/data/blob/master/css/at-rules.json, a schema error was detected for the `@property` object (https://github.com/mdn/data/blob/3a92ce8348e83a0ce7beef84b45accf151645f92/css/at-rules.json#L331):
```json
"@property": {
"syntax": "@property {\n \n}",
"status": "experimental",
"mdn_url": "https://developer.mozilla.org/docs/Web/CSS/@property"
},
```
because it was missing the `groups` property per https://github.com/mdn/data/blob/3a92ce8348e83a0ce7beef84b45accf151645f92/css/at-rules.schema.json#L39
Although the error output reports `JSON Schema – 1 error(s)`, the detailed error message is blank:
```
...
css/at-rules.json
Style – OK
JSON Schema – 1 error(s)

css/at-rules.schema.json
Style – OK
...
```

This seems to be a problem in the `better-ajv-errors` npm package.

NOTE (to self): The value of `ajv.errors` at https://github.com/mdn/data/blob/3a92ce8348e83a0ce7beef84b45accf151645f92/test/lint.js#L75 was:
```json
[
{
keyword: 'required',
dataPath: '/@property',
schemaPath: '#/additionalProperties/required',
params: { missingProperty: 'groups' },
message: "should have required property 'groups'"
}
]
```

Start with test/lint.js at the reported line and reproduce linting css/at-rules.json against css/at-rules.schema.json. Inspect how the recorded ajv.errors entry is passed to better-ajv-errors; done means the existing missing-property error is rendered with a non-blank detailed message.

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