BaseBuilder variable type fix

[TysiacSzescset]

Description
Changed $this->delete call second parameter from empty string to null.
In delete method declaration second param is type of integer or null.
IMHO it is better not to rely on type juggling where it is not necessary.

Checklist:

• Securely signed commits
• Component(s) with PHPdocs
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide