fix: @return in filter.tpl.php

[kenjis]

Description
See #6310

• fix @return in filter.tpl.php

Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• [] Unit testing, with >80% coverage
• [] User guide updated
• Conforms to style guide

[michalsn]

Why null is not an option?

[kenjis]

Because there is not null in the interface.

CodeIgniter4/system/Filters/FilterInterface.php

Line 34 in 6c187dd

* @return RequestInterface|ResponseInterface|string|void

The fewer types the better. is there any reason why we need null?

[MGatner]

null/void is kind of a bogus return type distinction in PHP. I think matching the interface is the way to go.

[michalsn]

The fewer types the better. is there any reason why we need null?

Nah, I haven't checked the interface. Sounds good. I probably saw null somewhere else.