Fix empty fragment string

[aguingand]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and discussions and couldn’t find anything or linked relevant results below
• I made sure the docs are up to date
• I included tests (or that’s not needed)

Description of changes

I've come to an error trying to stringify an empty string when fragment: true is enabled.
It fails here because hast-util-to-dom returns an XmlDocument with documentElement: null. So there is 2 solutions to me, either :

1. Handle the XmlDocument with documenElement: null in rehype-dom-stringify
2. Or, always return a DocumentFragment in hast-util-to-dom even if there is no child nodes

I have done solution 1 in this PR along with a failing test. If it you think solution 2 is more appropriate, I can make a PR to hast-util-to-dom.

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All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (6d41f5f) to head (2f91bc5).
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Additional details and impacted files

@@            Coverage Diff            @@
##              main       #26   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            5         5           
  Lines          102       135   +33     
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+ Hits           102       135   +33     

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[wooorm]

Thanks!

Not sure about 2. Perhaps? It sounds sensical as you say to return a fragment? And weird if it doesn’t?

[wooorm]

released, thank you!

[aguingand]

hast-util-to-dom always returning a DocumentFragment if fragment = true seems more consistent to me !

[wooorm]

Perhaps this is the mistake? https://github.com/syntax-tree/hast-util-to-dom/blob/5f2adac8060e43669e14d88624267de6a569f9f0/lib/index.js#L141. I’d welcome an improvement there if you want to look at it!