minor symfony#20717 Add missing default value in the Twig reference (stof)

This PR was merged into the 6.4 branch.

Discussion
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Add missing default value in the Twig reference

This also fixes the rendering, because empty literals are not valid in rST so this was displaying 4 backticks:

![image](https://github.com/user-attachments/assets/47ea8b1a-823e-4426-aa74-cc3430c44ebd)

Commits
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c35fb93 Add missing default value in the Twig reference

Author: OskarStark

reference/configuration/twig.rst

@@ -315,7 +315,7 @@ mailer
 html_to_text_converter
 ......................
 
-**type**: ``string`` **default**: ````
+**type**: ``string`` **default**: ``null``
 
 .. versionadded:: 6.2