docutils: docutils.nodes.Node.next_node can return None

[adamtheturtle]

See code at https://github.com/docutils/docutils/blob/6d835be25d585e1dce688ef33c3876aa34ba59f0/docutils/docutils/nodes.py#L369:

    def next_node(self,
                  condition: type | Callable[[Node], bool] | None = None,
                  include_self: bool = False,
                  descend: bool = True,
                  siblings: bool = False,
                  ascend: bool = False,
                  ) -> Node | None:
        """
        Return the first node in the iterator returned by findall(),
        or None if the iterable is empty.

        Parameter list is the same as of `findall()`.  Note that `include_self`
        defaults to False, though.
        """
        try:
            return next(self.findall(condition, include_self,
                                     descend, siblings, ascend))
        except StopIteration:
            return None

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

sphinx (https://github.com/sphinx-doc/sphinx)
+ sphinx/writers/html5.py: note: In member "depart_classifier" of class "HTML5Translator":
+ sphinx/writers/html5.py:477:27: error: Incompatible types in assignment (expression has type "Node | None", variable has type "Node")  [assignment]
+ sphinx/writers/html5.py: note: In member "depart_term" of class "HTML5Translator":
+ sphinx/writers/html5.py:488:27: error: Incompatible types in assignment (expression has type "Node | None", variable has type "Node")  [assignment]
+ sphinx/writers/latex.py: note: In member "visit_target" of class "LaTeXTranslator":
+ sphinx/writers/latex.py:1855:25: error: Incompatible types in assignment (expression has type "Node | None", variable has type "Node")  [assignment]
+ sphinx/writers/texinfo.py:293: error: Unused "type: ignore" comment  [unused-ignore]
+ sphinx/builders/_epub_base.py: note: In member "fix_ids" of class "EpubBuilder":
+ sphinx/builders/_epub_base.py:297:31: error: Incompatible types in assignment (expression has type "Node | None", variable has type "Node")  [assignment]