Pharo7+: contents of empty binary file are `nil` rather than an empty instance of ByteArray

Given an empty file `somefile`, the following returns `nil` instead of an empty instance of `ByteArray`.

```Smalltalk
GRPlatform current contentsOfFile: 'somefile' binary: true
```
This is not consistent with text files (which return an empty instance of `String`), nor with previous Pharo versions.

The change is caused because of https://github.com/SeasideSt/Grease/pull/106