Fix: Allow null block names in core block filter

[alecgeatches]

Description

Fixes #99.

This is a follow-up PR for #100 to fix CI tests from phpcs failures, and to add a comment. See original description in #100:

This updates the synced-pattern inner-block filter to accept the null block names that WordPress can pass for parsed non-block/classic-content fragments. In that case the filter now returns the existing inner blocks unchanged instead of raising a PHP TypeError before the parser can continue.

I also added a focused regression test that calls the filter with a null block name.

Verification: I could not run the WordPress/PHP test suite in this cron profile because php/composer/git are not installed here. I did a static patch check against the current trunk file contents via the GitHub API.