`$.attr('value')` returns incorrect result for an `<option>` without a value attribute

## Steps to reproduce

```js
const cheerio = require('cheerio')

const $ = cheerio.load(`

Red
Green
Blue

`)

console.log($('option:first-child').attr('value'))
```

## Actual result

'Red' is logged to the console, which is the inner text of the option

## Expected result

`undefined` should be logged to the console, because the element does not have a `value` attribute.

This would match how `attr` works both [according to jQuery's documentation](https://api.jquery.com/attr/) and when compared to [it's actual behaviour (CodePen)](https://codepen.io/36degrees/pen/QWJwvJG?editors=1111).

## Further details

It looks like this may be caused by this code in the `getAttr`function:

https://github.com/cheeriojs/cheerio/blob/273789820ec281defd6854ab13df8f9fb9caaafc/src/api/attributes.ts#L64-L67

This was introduced as part of #671 but should probably have only affected the `val` function [as described in the issue](https://github.com/cheeriojs/cheerio/issues/633) it was trying to fix.

I'd be open to raising a pull request with a fix if that would be helpful.

Start in src/api/attributes.ts at the getAttr implementation around lines 64–67, then reproduce the option example from the issue. Review the behavior introduced by #671 and the context in #633; done means attr('value') returns undefined when the attribute is absent while the related val behavior remains correct.

Written by the indexing model from the issue text.