proportional resizing of images not working as described in docs

The docs at [https://docs.bolt.cm/3.2/templating/templatetags#image](https://docs.bolt.cm/3.2/templating/templatetags#image) state:

>To scale an image proportionally to a given width or height, set the other dimension to null, and set cropping mode to resize.
>```
>
>```

This is not working for me, it creates proportional images/thumbs of height 120px.
(120px is my default thumbnail-height in config.yml)
The image is places in `/thumbs/400x120r/filename.thumb.jpg`

The expected behaviour can be achieved by using:
```

```
(replacing *null* with *0*)
That creates a proportional image of width 400px which is placed in `/thumbs/400x0r/filename.thumb.jpg`

So either the docs should be updated or the image() and thumbnail() function should check for *null*

Details
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- Relevant Bolt Version: 3.2.12
- Install type: Zip/tarball install
- PHP version: 5.6.30
- Used web server: Apache/2.4.25 (Unix)