bp_core_get_member_display_name( string $display_name, int $user_id = null )

function bp_core_get_member_display_name( $display_name, $user_id = null ) {
	// some cases it calls the filter directly, therefore no user id is passed
	if ( ! $user_id ) {
		return $display_name;
	}

	$old_display_name = $display_name;

	$display_name = bp_xprofile_get_member_display_name( $user_id );

	if ( empty( $display_name ) ) {
		$display_name = $old_display_name;
	}

	return apply_filters( 'bp_core_get_member_display_name', trim( $display_name ), $user_id );
}