nixos-render-docs: fix empty inlines

`children` only has to be not-None, not also not-[]. empty table cells
would can produce an empty inline token and cause these checks to fail,
even though the token actually is totally valid.

showed up in
https://github.com/NixOS/nixpkgs/pull/246956#discussion_r1283146120.

pkgs/tools/nix/nixos-render-docs/src/nixos_render_docs/manual.py

@@ -623,7 +623,7 @@ class HTMLConverter(BaseConverter[ManualHTMLRenderer]):
             elif bt.type == 'footnote_ref' and (id := cast(str, bt.attrs.get('id', ''))):
                 result.append(XrefTarget(id, "???", None, None, target_file))
             elif bt.type == 'inline':
-                assert bt.children
+                assert bt.children is not None
                 result += self._collect_ids(bt.children, target_file, typ, False)
             elif id := cast(str, bt.attrs.get('id', '')):
                 # anchors and examples have no titles we could use, but we'll have to put

pkgs/tools/nix/nixos-render-docs/src/nixos_render_docs/md.py

@@ -458,7 +458,7 @@ def _footnote_ids(md: markdown_it.MarkdownIt) -> None:
                 token.attrs['id'] = f'{token.meta["label"]}.__back.{token.meta["subId"]}'
                 token.meta['target'] = token.meta["label"]
             elif token.type == 'inline':
-                assert token.children
+                assert token.children is not None
                 generate_ids(token.children)
 
     def footnote_ids(state: markdown_it.rules_core.StateCore) -> None: