leading_whitespace and trailing_newline don't play nicely on empty files

[amomchilov]

New Issue Checklist

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

Describe the bug

trailing_newline requires that a file ends with a single extra new-line character, even for empty files. Once this happens, leading_white space gets violated, because (as far as it sees it), the file now starts with a redundant new line.

To preempt the question of "Why do you have an empty file in the first place?": I just have a little empty playground I keep around in my project as a quick little scratch space, to use similar to the REPL, but with support for packages from my workspace.

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

$ swiftlint
Linting Swift files in current working directory
/path/dev playground.playground/Contents.swift:1:1: warning: Leading Whitespace Violation: File shouldn't start with whitespace: currently starts with 1 whitespace characters (leading_whitespace)
Done linting! Found 1 violation, 0 serious in 243 files.

Then when you try to fix it:

$ swiftlint --fix
Correcting Swift files in current working directory
/path/dev playground.playground/Contents.swift:1:1 Corrected Leading Whitespace
/path/dev playground.playground/Contents.swift:1:1 Corrected Trailing Newline

It actually hits a fixed point. Rerunning it over and over again doesn't cause any net changes to to the file.

Environment

• SwiftLint version: 0.46.2
• Installation method used: Homebrew
• Paste your configuration file:

excluded:
  - vendor
  - "**/.build"
disabled_rules:
# TODO – I eventually want to have these all fixed
  # I'm okay with force-casting for a lot of this stuff, because C interop often requires it.
  # Nonetheless, it would be good to add in an "unwrap or die" operator (https://github.com/apple/swift-evolution/blob/master/proposals/0217-bangbang.md),
  # to require the author to add an explanation as to why force-casting is safe in each instance.
  - force_cast
  - force_try
  - todo # Lol, I have a TODO to fix the TODOs
# These rules are disabled because I like to manually line things up where it adds clarity
  - colon
  - comma
# Other
  # I would only like to enforce this rule for non-test files, but it doesn't look like
  # exclusion patterns on a per-rule basis: https://github.com/realm/SwiftLint/issues/2355
  - function_body_length
  # I think short variable names are perfectly acceptable in the right contexts
  - identifier_name
  # Meh, deep nesting is useful sometimes
  - nesting
# Disabling these rules so I can apply auto-corrections for them 1 by 1, so that it's easier to review the changes.
  - unused_closure_parameter
  - unused_optional_binding
  - unused_setter_value
  - vertical_parameter_alignment
  - vertical_whitespace
  - void_return

trailing_comma:
  mandatory_comma: true

type_name:
  allowed_symbols: ["_"] # _ can be useful in Test/Spec type names.
  excluded: ["ID"]

• Are you using nested configurations? Nope
• Which Xcode version are you using (check xcodebuild -version)? Xcode 13.2.1 Build version 13C100
• 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.

echo "\n" | swiftlint lint --no-cache --use-stdin --enable-all-rules
<nopath>:1:1: warning: Leading Whitespace Violation: File shouldn't start with whitespace: currently starts with 2 whitespace characters (leading_whitespace)
<nopath>:2:1: warning: Trailing Newline Violation: Files should have a single trailing newline. (trailing_newline)
<nopath>:2:1: warning: Vertical Whitespace Violation: Limit vertical whitespace to a single empty line. Currently 2. (vertical_whitespace)
Done linting! Found 3 violations, 0 serious in 1 file.