[Workbench] `json_decode` an invalid `composer.json` file will return `null` insteads of `array`

[crynobone]

We should consider throwing an exception when this happened:

https://github.com/orchestral/testbench-core/blob/b401efab5a5223a41d355daf7efc83a7615c9632/src/Workbench/Composer.php#L13-L25