chore: allow outputting to folder if its empty

[iTrooz]

Very small PR, just allows you to output to a directory that is empty :)

[Copilot]

Pull request overview

This PR updates ApkDecoder.decode(File outDir) to allow decoding into an output directory that already exists as long as it is empty, aligning behavior with the PR’s intent of permitting “empty folder” outputs without requiring -f.

Changes:

• Adjust output-directory existence check to permit existing empty directories.
• Continue to block decoding into non-empty output directories unless forced.

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[IgorEisberg]

The test is an overkill. You're also using a completely unrelated test issue2701.apk for this. That's confusing.
I'm not a fan of using File.listFiles for this, it's heavy and we're only interested in stopping after first match, not allocating a potentially huge array of files.
There's a better way to achieve this, one of them is deleting the folder using rmfile after checking that it's a directory and not a symlink, then rechecking if it still exists. (If the folder was not empty, rmfile fails silently).
The other option is using a DirectoryStream and just calling stream.iterator().hasNext(). This immediately stops after first match.