[6.x] Add assertion for empty job chain

[iamgergo]

This PR introduces the assertPushedWithEmptyChain assertion.

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I found myself to implement conditional job chaining:

MoveFile::withChain(
    $medium->isImage ? [new GenerateImages($medium)] : [],
)->dispatch($medium);

It's possible to pass an empty chain, but right now there is no way to test it.

The assertPushedWithChain does not accept an empty array for the expected chain and the assertPushed can't tell whether the chain is empty or not.

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I'm not sure, this is the cleanest approach, I'm up to any idea.