Add null safety checks in forums code

[GaryJones]

Problem

Three places in the forums code lack null safety checks, causing PHP warnings, deprecation notices, or potential fatal errors on PHP 8.1+.

1. Null post object access in forum group detection

In bp-forums/functions.php, get_post( $forum_id ) can return null when the forum post does not exist (e.g. it was deleted or the ID is invalid). The code immediately accesses $current_post->post_parent without checking, causing a fatal error.

Fix: Add $current_post && before accessing the property.

2. REST reply permission check logic error

In class-bp-rest-reply-endpoint.php, the update_item_permissions_check method calls $this->get_item_permissions_check() which may return a WP_Error. The code then unconditionally compares the reply author, even when $retval is already an error. This can cause unexpected behaviour or mask the original permission error.

Fix: Add true === $retval && to only evaluate the author check when the prior permission check passed.

3. Null passed to sanitize_title() for REST topic tag

In class-bp-rest-topics-endpoint.php, $request->get_param( 'tag' ) returns null when the parameter is not provided. Passing null to sanitize_title() triggers a deprecation warning on PHP 8.1+.

Fix: Add ?? '' null-coalescing fallback.

Test plan

• Access a forum where the forum post has been deleted — should not fatal
• Make an unauthenticated REST API request to update a reply — should return a proper permission error
• Make a REST API request to list topics without a tag parameter on PHP 8.1+ — no deprecation warning

[chetansatasiya]

@GaryJones Thanks for the PR and it will be added in our upcoming release.