eZDFSFileHandlerMySQLiBackend: Small code enhancements

I stumbled on this while reviewing #808, it's really minor but enhances readability and reduces code complexity (if you use such tools).

1. `0` is returned in the precedent `if` statement, so the `else` statement is not needed
2. The `$files` variable was used inside the loop without having it defined beforehand.

Cheers
:octocat: Jérôme

PS: Two commits because two issues, but I can of course merge them if you want.