[dart/en-us] Update dart.html.markdown

[aka-radix]

Add a comment regarding using the default List constructor which is not available when null safety is enabled, and a short explanation of null safety.

• I solemnly swear that this is all original content of which I am the original author
• Pull request title is prepended with [language/lang-code] (example [python/fr-fr] or [java/en])
• Pull request touches only one file (or a set of logically related files with similar changes made)
• Content changes are aimed at intermediate to experienced programmers (this is a poor format for explaining fundamental programming concepts)
• If you've changed any part of the YAML Frontmatter, make sure it is formatted according to CONTRIBUTING.md
  • Yes, I have double-checked quotes and field names!

[verhovsky]

Dart 3 enables null safety so instead of this comment, this code should be updated to work or removed, like in #4673