Performance/Sum should avoid, or warn about, empty arrays

[ghiculescu]

Fixes #177

Doesn't fully resolve the issue, since it only works on array literals. But this removes at least one false positive.

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[marcandre]

I can't see anyone ever writing [].inject(...)

[ghiculescu]

I can't see anyone ever writing [].inject(...)

No, it's more a starting point for the broader fix. I should have marked this PR as a draft.

[ghiculescu]

Okay, I don't think rubocop can know if an array is empty unless it's an array literal. So I added a warning to the error message for cases that look like #177. They already don't autocorrect.

[koic]

Thanks!