Using an equal sign (or not) with an array option changes the value

When using the following code:

```js
const yargs = require('yargs')
console.log(yargs.option('opt', { array: true }).parse().opt)
```

Using a `=` or not changes the parsed value, which is quite confusing:

```bash
$ node yargs.js --opt=""
[]

$ node yargs.js --opt ""
[ '' ]
```

Using yargs `16.2.0` on Node `15.6.0`.