cherry-pick valuable updates from original repo

[brodybits]

https://github.com/jindw/xmldom/pulls

since the original repo is no longer supported

• X3-88886: to avoid empty namespace value like xmlns:ds="" jindw/xmldom#236 => fix!: Avoid empty namespace value like xmlns:ds="" #168
• Fix "]]>" serialization in text nodes jindw/xmldom#207 => fix(dom): Escape ]]> when serializing CharData #181
• ... add a comment mentioning the PR you would like to port, so we can include it here ...

[karfau]

Some have already been included but it's a good way to find issues that users are already aware of.
I think whenever a PR is integrated into this repo we should go back to the original PR and link to the PR that includes the change.
Alternatively we could maintain a list of included PRs here, but I'm not sure how helpful it is to find the ones that have not been ported...

Or the other way around: When anybody finds a PR that could be worthwhile to include please mention it here and we will maintain a list of the ones we want to have ported in the description.