Checkstyle formatter does not include error free files.

[mastermatt]

Bug Report

• TSLint version: 5.9.1
• TypeScript version: 2.8.1
• Running TSLint via: CLI

TypeScript code being linted

// error free code

with tslint.json configuration:

{
    "extends": [
        "tslint:recommended"
    ]
}

tslint --project . --format checkstyle index.ts

Actual behavior (newlines added for readability)

<?xml version="1.0" encoding="utf-8"?>
<checkstyle version="4.3">
</checkstyle>

Expected behavior

<?xml version="1.0" encoding="utf-8"?>
<checkstyle version="4.3">
<file name="/path/to/src/index.ts"></file>
</checkstyle>

As noted in #250, this file is the closest thing to an official spec on the format.
The relevant bit is that writeFileMessages writes a tag for each file linted. Errorless files are just empty tags.
As I'm finding out, some parsers need those tags. Otherwise they return an error message stating no lint checks were preformed and/or they allows show 100% of files have issues.

I opted to open an issue on this instead of submitting a PR as I'm unsure of the preferred way to handle this. The Linter class doesn't store all the files it lints and therefore can't pass them to the formatter in getResults() as is. One option would be to just add each file in turn in lint(), then add a third arg to IFormatter.format.

[mastermatt]

Seems related to #3744

[mastermatt]

This was resolved with #3838