Move to strict mode and fix tests

src/Monolog/Handler/DeduplicationHandler.php

@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 /*
  * This file is part of the Monolog package.
@@ -144,7 +144,7 @@ class DeduplicationHandler extends BufferHandler
 
         while (!feof($handle)) {
             $log = fgets($handle);
-            if (substr($log, 0, 10) >= $timestampValidity) {
+            if ($log && substr($log, 0, 10) >= $timestampValidity) {
                 $validLogs[] = $log;
             }
         }