Do not use an optional parameter in LintMessage's constructor (#97)

* Do not use an optional parameter in LintMessage's constructor

Using optional parameters before required parameters is deprecated since PHP 8.0 -- https://www.php.net/manual/en/migration80.deprecated.php

Removing the default value has no effect.

* Update the parameter type for `$file`

It might also be `null`

PhpcsChanged/LintMessage.php

@@ -9,7 +9,7 @@ class LintMessage {
 	private $type;
 	private $otherProperties;
 
-	public function __construct(int $line, string $file = null, string $type, array $otherProperties) {
+	public function __construct(int $line, ?string $file, string $type, array $otherProperties) {
 		$this->line = $line;
 		$this->file = $file;
 		$this->type = $type;