Increase PHPStan levels and update code to resolve issues found

tests/Codec/StringCodecTest.php

@@ -44,9 +44,7 @@ class StringCodecTest extends TestCase
     protected function tearDown(): void
     {
         parent::tearDown();
-        $this->builder = null;
-        $this->uuid = null;
-        $this->fields = null;
+        unset($this->builder, $this->uuid, $this->fields);
     }
 
     public function testEncodeUsesFieldsArray()