`<link preload>` is not inserted when using OptimizeHeroImages(PHP 5.6)

```
amp-toolbox-version : 0.11.0, 0.10.0, 0.6.0
PHP version : 5.6
```

`` may not be inserted when using `Transformer\OptimizeHeroImages` with ``.
I also use `Transformer\ServerSideRendering`.

Could you check this problem?

Reproduce the issue with PHP 5.6 using Transformer\OptimizeHeroImages and Transformer\ServerSideRendering on an amp-img data-hero input, starting from those transformer entry points. Compare the generated output across the listed amp-toolbox versions; done means the expected is inserted consistently when a hero image is present.

Written by the indexing model from the issue text.