Avoid potential PHP errors when using BuddyPress buttons on front end

[imath]

[13299], [13300], [13301], [13302] & [13303] introduced a potential regression if the corresponding button arguments filter is used by some custom code like:

add_filter( 'bp_get_send_public_message_button', '__return_false' );

This was reported @fantamily by here: ​https://buddypress.org/support/topic/say-hi-to-buddypress-11-0-0-la-scala/#post-328004

Returning false to one the button arguments filter has two different effects depending on the active template pack:

• Legacy -> fatal error (Uncaught TypeError)
• Nouveau -> Warning (Trying to access array offset on value of type bool)

Trac ticket: ​https://buddypress.trac.wordpress.org/ticket/8800

[imath]

In 13409:

Prevent a Members single view header action button filters wrong use

Some custom code/plugins might wrongly use something else than an array
(eg: false) to disable one of the action buttons shown inside the single
Members view header. Changes made during the 11.0.0 release revealed this
potential issue.

See #7126
See #8800 (Trunk)

[imath]

In 13410:

Prevent a Members single view header action button filters wrong use

Some custom code/plugins might wrongly use something else than an array
(eg: false) to disable one of the action buttons shown inside the single
Members view header. Changes made during the 11.0.0 release revealed this
potential issue.

See #7126
Closes ​https://github.com/buddypress/buddypress/pull/59
Fixes #8800 (branch 11.0)