Fix TypeError bug where modifiedFiles is null

SourceFileProcessor::$modifiedFiles ends up being null if the list of
modified files is empty. Since it is null, and not an empty string, this
would throw a TypeError because null is not an array.

The fix is to initialize SourceFileProcessor::$modifiedFiles on object
construction.

Author: tomzx

src/PHPSemVerCheckerGit/SourceFileProcessor.php

@@ -39,7 +39,7 @@ class SourceFileProcessor
     /**
      * @var string[]
      */
-    private $modifiedFiles;
+    private $modifiedFiles = [];
 
     /**
      * SourceFileProcessor constructor.