This commit fixes the following PHPStan errors introduced after the update to version 0.12.83:
```
------ ----------------------------------------------------------------------
Line lib/Newsletter/Renderer/Blocks/Footer.php
------ ----------------------------------------------------------------------
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]
- Removes background color when it's set to "transparent"
- Removes second line break from paragraphs
- Introduces other changes based on Becks's testing
- Updates unit tests