fix: multiline styling stuck/not working properly

[Wrth1]

Description

This PR fixes the bug mentioned in #1550 , where doing styling in multiple lines at the same time doesn't change the toggle. This bug makes the editor so unintuitive.

Related Issues

Fix #1550

Improvements

This change enables us to do styling to multiple line at once without breaking the style toggle, (see #1550 for examples)

Additional notes

The issue was at lib\src\models\documents\nodes\line.dart at line 364-367, it compares style.attributes[attr.key] != attr.value when it should actually compare style.attributes[attr.key]?.value != attr.value. At the issue you can see that @li3317 cannot reproduce it, it might be because in some version of dart anything == (bool)true while in some version only (bool)true == (bool)true

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I titled the PR using Conventional Commits.
• I did not modify the CHANGELOG.md nor the plugin version in pubspec.yaml files.
• All existing and new tests are passing.
• I have run the commands in ./scripts/before_push.sh and it all passed successfully

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate that with a ! in the title as explained in Conventional Commits).
• No, this is not a breaking change.