Fix error_handler return type declaration

tests/Monolog/Formatter/NormalizerFormatterTest.php

@@ -182,6 +182,8 @@ class NormalizerFormatterTest extends \PHPUnit\Framework\TestCase
                 restore_error_handler();
                 $that->fail("$message should not be raised");
             }
+
+            return true;
         });
 
         $formatter = new NormalizerFormatter();
@@ -215,6 +217,8 @@ class NormalizerFormatterTest extends \PHPUnit\Framework\TestCase
                 restore_error_handler();
                 $that->fail("$message should not be raised");
             }
+
+            return true;
         });
 
         $formatter = new NormalizerFormatter();

tests/Monolog/Handler/RotatingFileHandlerTest.php

@@ -34,6 +34,8 @@ class RotatingFileHandlerTest extends TestCase
                 'code' => $code,
                 'message' => $message,
             ];
+
+            return true;
         });
     }