Update Low level HTML articles to null safety (#3860)

* Update Connect Dart and HTML article to null safety

* Update Add elements to the DOM tutorial to new safety

* Update Remove DOM elements tutorial to null safety

* Optimize existing diagrams and images

* Fix code block language

* Add another null safety possibility

* Correctly use semicolon instead of period

* Adjust nullable element section

diagrams/tutorials/web/low-level-html/add-element-code.drawio

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diagrams/tutorials/web/low-level-html/css-property-value.drawio

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diagrams/tutorials/web/low-level-html/css-rule-explained.drawio

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diagrams/tutorials/web/low-level-html/dart-html-connect.drawio

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diagrams/tutorials/web/low-level-html/event-listener-exp.drawio

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diagrams/tutorials/web/low-level-html/paragraph-dom.drawio

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diagrams/tutorials/web/low-level-html/remove-all-code.drawio

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diagrams/tutorials/web/low-level-html/remove-element-code.drawio

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diagrams/tutorials/web/low-level-html/simple-dom-tree.drawio

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