False positive in empty_string with multiquote literals

[marcelofabri]

New Issue Checklist

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

Describe the bug

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

$ swiftlint lint

Environment

• SwiftLint version (run swiftlint version to be sure)? 0.38.2
• Installation method used (Homebrew, CocoaPods, building from source, etc)?
• Paste your configuration file:

# insert yaml contents here

• Are you using nested configurations?
  If so, paste their relative paths and respective contents.
• Which Xcode version are you using (check xcodebuild -version)? 11.3.1
• 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.

let str = """
Eog==
"""