Fixes #115: Resolve undefined 'link_class' warning for group join button

[anas-ha]

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Changes proposed in this Pull Request:

This PR fixes an issue where a warning (Undefined array key "link_class") is triggered in BuddyBoss groups, caused by the absence of the "link_class" key in PHP 8+. The fix uses a null coalescing operator (??) to provide a default empty value, preventing the warning and maintaining compatibility.

Additionally, the order of conditions within the if statement in the callback function of 'bp_get_group_join_button' (pmpro_bp_disable_group_buttons) was corrected, and parentheses were added to ensure the correct evaluation of conditions. The updated condition checks if the user can join the group first before evaluating the presence and value of $button_args['id'], enhancing both readability and accuracy.

How to test the changes in this Pull Request:

1. Set up a BuddyBoss group with the Paid Memberships Pro and Paid Memberships Pro - BuddyPress and BuddyBoss Integration.
2. Navigate to the group.

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Changelog entry

Fixes PHP 8+ warning for undefined array key "link_class" in the group join button when group joining is restricted. Adjusted the condition order and added parentheses for clarity and correctness in evaluating conditions.