Hi,

While working on upgrading some dependencies, we have been testing an upgrade of this package from 2.0.1 to 3.0.2, and came across the following behavior which appears to be a regression. Can you please investigate and confirm whether this is intended or not?

Expected Behavior / Behavior in 2.0.1
Diffing two identical one-line strings, such as 'these strings contain no differences', and using the UnifiedDiffOutputBuilder returns an empty string, i.e. a string with length 0 ("").

Observed Behavior in 3.0.2
Diffing two identical one-line strings returns the string "\n", i.e. a one-byte length string with the newline character in it.

Reproduction
Using composer to manage the dependency, the following test harness file can enable you to quickly test between different versions.

<?php
require_once('vendor/autoload.php');

use SebastianBergmann\Diff\Differ;
use SebastianBergmann\Diff\Output\UnifiedDiffOutputBuilder;

$a = 'something that is the same';
$b = 'something that is the same';

$differ = new Differ(new UnifiedDiffOutputBuilder(''));
$diff = $differ->diff($a, $b);

echo "The diff is:\n";
echo serialize($diff);

Running the above script with composer pulling in sebastian/diff version 2.0.1 outputs:

The diff is:
s:0:""

Whereas running the script with sebastian/diff 3.0.2 outputs:

The diff is:
s:1:"
"

We are using this library to detect differences and treat any non-zero length output as a "difference". I suspect other consumers of this library likely do the same.

Note also that using the command-line diff tool on two identical files on linux outputs a zero-length response as well, so the expected behavior is consistent with built-in tools on common platforms.