Fix the issues reported by PHP_CodeSniffer

Running phpcs --standard=Drupal,DrupalPractice --extensions=php,inc,module,install,info,test,profile,theme modules/custom/codesnippet/ shows the following warnings/errors, which should be fixed.

FILE: src/Plugin/CKEditorPlugin/CodeSnippet.php
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FOUND 1 ERROR AND 2 WARNINGS AFFECTING 3 LINES
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 74 | WARNING | [ ] t() calls should be avoided in classes, use
    |         |     \Drupal\Core\StringTranslation\StringTranslationTrait and $this->t() instead
 79 | WARNING | [ ] t() calls should be avoided in classes, use
    |         |     \Drupal\Core\StringTranslation\StringTranslationTrait and $this->t() instead
 95 | ERROR   | [x] Use null coalesce operator instead of ternary operator.
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PHPCBF CAN FIX THE 1 MARKED SNIFF VIOLATIONS AUTOMATICALLY
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