New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

Describe the bug

The vertical_whitespace_between_cases rule doesn't trigger when using @unknown before default in a switch.

Complete output when running SwiftLint, including the stack trace and command used

> ./swiftlint lint
Linting Swift files in current working directory
Linting 'Test2.swift' (1/2)
Linting 'Test1.swift' (2/2)
/Users/revolt/Downloads/swiftlint-unknown/Test1.swift:11:1: warning: Vertical Whitespace Between Cases Violation: Include a single empty line between switch cases. (vertical_whitespace_between_cases)
/Users/revolt/Downloads/swiftlint-unknown/Test2.swift:11:1: warning: Vertical Whitespace Between Cases Violation: Include a single empty line between switch cases. (vertical_whitespace_between_cases)
/Users/revolt/Downloads/swiftlint-unknown/Test2.swift:13:1: warning: Vertical Whitespace Between Cases Violation: Include a single empty line between switch cases. (vertical_whitespace_between_cases)
Done linting! Found 3 violations, 0 serious in 2 files.

Environment

• SwiftLint version (run swiftlint version to be sure)? 0.43.0-rc.2
• Installation method used (Homebrew, CocoaPods, building from source, etc)? https://github.com/realm/SwiftLint/releases/download/0.43.0-rc.2/portable_swiftlint.zip
• Paste your configuration file:

opt_in_rules:
    - vertical_whitespace_between_cases

• Are you using nested configurations? No
  If so, paste their relative paths and respective contents.
• Which Xcode version are you using (check xcodebuild -version)? Xcode 12.2 Build version 12B45b
• Do you have a sample that shows the issue? Run echo "[string here]" | swiftlint lint --no-cache --use-stdin --enable-all-rules
  to quickly test if your example is really demonstrating the issue. If your example is more
  complex, you can use swiftlint lint --path [file here] --no-cache --enable-all-rules.

Test1.swift:

enum Test {
    case first
    case second
}

let test = Test.first

switch test {
case .first:
    print("first")
case .second:
    print("second")
@unknown default:
    print("default")
}

Test2.swift (that works correctly):

enum Test {
    case first
    case second
}

let test = Test.first

switch test {
case .first:
    print("first")
case .second:
    print("second")
default:
    print("default")
}