Return false for isFile for empty strings

[keith]

As of Swift 5, corelibs-foundation crashes if you pass an empty string
to fileExists https://github.com/apple/swift-corelibs-foundation/blob/cae5eaca63cd57f64a93b4d5de87a4ffc04466d5/Foundation/FileManager.swift#L2033

[marcelofabri]

Why are we passing "" in the first place? 🤔

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[keith]

It looks like that's the default if no path is passed

SwiftLint/Source/swiftlint/Helpers/CommonOptions.swift

Lines 4 to 8 in dd39c43

	func pathOption(action: String) -> Option<String> {
	return Option(key: "path",
	defaultValue: "",
	usage: "the path to the file or directory to \(action)")
	}

[keith]

Maybe that default should be a . though?

[marcelofabri]

Maybe that default should be a . though?

We can investigate and change, but I'm a bit afraid because we don't have unit tests for the swiftlint target. IMO we can merge this as is and then follow up.

[jpsim]

Fix looks good to me. Added a changelog entry in 9cc97d6. Will merge when CI passes.