interpret a None-env value as empty string

This commit interprets a None environment value
as an empty string. It also ensures that all
values are strings instead of, for example,
`int`.

It adds a paragraph to the doc-string of
`datalad_next.config.utils.set_gitconfig_items_in_env`
to document its behavior if a configuration item
has a `None`-value.

This commit is unrelated to Windows-support,
but became necessary because the datalad-next CI
runs datalad tests now always with the loaded
`datalad_next`-extension, and the previous version
of datalad_next's config code failed the tests.

datalad_next/config/utils.py

@@ -73,6 +73,12 @@ def set_gitconfig_items_in_env(items: Mapping[str, str | Tuple[str, ...]]):
 
     Multi-value configuration keys are supported (values provided as a tuple).
 
+    Any item with a value of ``None`` will be posted into the ENV with an
+    empty string as value, i.e. the corresponding ``GIT_CONFIG_VALUE_{count}``
+    variable will be an empty string. ``None`` item values indicate that the
+    configuration key was unset on the command line, via the global option
+    ``-c``.
+
     No verification (e.g., of syntax compliance) is performed.
     """
     _clean_env_from_gitconfig_items()
@@ -83,7 +89,7 @@ def set_gitconfig_items_in_env(items: Mapping[str, str | Tuple[str, ...]]):
         values = value if isinstance(value, tuple) else (value,)
         for v in values:
             environ[f'GIT_CONFIG_KEY_{count}'] = key
-            environ[f'GIT_CONFIG_VALUE_{count}'] = v
+            environ[f'GIT_CONFIG_VALUE_{count}'] = '' if v is None else str(v)
             count += 1
     if count:
         environ['GIT_CONFIG_COUNT'] = str(count)