This commit fixes the following PHPStan errors introduced after the update to version 0.12.83:
```
------ ----------------------------------------------------------------------
Line lib/Newsletter/Renderer/Blocks/Footer.php
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18 Parameter #1 $html of method MailPoet\Util\pQuery\pQuery::parseStr()
expects string, array|string|null given.
------ ----------------------------------------------------------------------
------ ----------------------------------------------------------------------
Line lib/Newsletter/Renderer/Blocks/Header.php
------ ----------------------------------------------------------------------
18 Parameter #1 $html of method MailPoet\Util\pQuery\pQuery::parseStr()
expects string, array|string|null given.
------ ----------------------------------------------------------------------
```
[MAILPOET-3491]