Fix the issues reported by phpcs

Problem/Motivation

Coding Standard
htmlmail/src/Plugin/Mail/HtmlMailSystem.php
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FOUND 6 ERRORS AFFECTING 6 LINES
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81 | ERROR | [ ] Missing member variable doc comment
82 | ERROR | [ ] Missing member variable doc comment
83 | ERROR | [ ] Missing member variable doc comment
293 | ERROR | [x] list(...) is forbidden, use [...] instead.
332 | ERROR | [x] list(...) is forbidden, use [...] instead.
472 | ERROR | [ ] The array declaration extends to column 110 (the limit is 80). The array content should be split up over multiple lines
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PHPCBF CAN FIX THE 2 MARKED SNIFF VIOLATIONS AUTOMATICALLY
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htmlmail/src/Helper/HtmlMailHelper.php
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FOUND 2 ERRORS AFFECTING 2 LINES
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6 | ERROR | [x] Non-namespaced classes/interfaces/traits should not be referenced with use statements
73 | ERROR | [x] Use null coalesce operator instead of ternary operator.
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PHPCBF CAN FIX THE 2 MARKED SNIFF VIOLATIONS AUTOMATICALLY
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htmlmail/src/Utility/HtmlMailMime.php
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FOUND 3 ERRORS AFFECTING 3 LINES
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573 | ERROR | [x] Use null coalesce operator instead of ternary operator.
581 | ERROR | [x] Use null coalesce operator instead of ternary operator.
583 | ERROR | [x] Use null coalesce operator instead of ternary operator.
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Steps to reproduce

vendor/bin/phpcs --standard="Drupal,DrupalPractice" -n --extensions="php,module,inc,install,test,profile,theme" htmlmail/

Data model changes