Fix issue identified by Psalm

Author: sebastianbergmann

src/Framework/TestCase.php

@@ -275,12 +275,12 @@ abstract class TestCase extends Assert implements Reorderable, SelfDescribing, T
     private $output = '';
 
     /**
-     * @var string
+     * @var ?string
      */
     private $outputExpectedRegex;
 
     /**
-     * @var string
+     * @var ?string
      */
     private $outputExpectedString;
 
@@ -305,7 +305,7 @@ abstract class TestCase extends Assert implements Reorderable, SelfDescribing, T
     private $outputRetrievedForAssertion = false;
 
     /**
-     * @var Snapshot
+     * @var ?Snapshot
      */
     private $snapshot;