Fix the issues reported by phpcs

Problem/Motivation

Getting following errors.

FILE: /var/www/html/modules/contrib/menu_pager/src/Plugin/Block/MenuBlock.php
-----------------------------------------------------------------------------
FOUND 5 ERRORS AFFECTING 5 LINES
-----------------------------------------------------------------------------
108 | ERROR | [x] Use null coalesce operator instead of ternary operator.
115 | ERROR | [x] Use null coalesce operator instead of ternary operator.
122 | ERROR | [x] Use null coalesce operator instead of ternary operator.
132 | ERROR | [x] Use null coalesce operator instead of ternary operator.
146 | ERROR | [x] Use null coalesce operator instead of ternary operator.
-----------------------------------------------------------------------------
PHPCBF CAN FIX THE 5 MARKED SNIFF VIOLATIONS AUTOMATICALLY
-----------------------------------------------------------------------------

Time: 2.35 secs; Memory: 6MB

Steps to reproduce

Run following command

phpcs --standard=Drupal,DrupalPractice --extensions=php,module,inc,install,test,profile,theme,css,info,txt,md,yml modules/contrib/menu_pager/

Proposed resolution

Above errors need to be fixed.