interpret 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`.

datalad_next/config/utils.py

@@ -83,7 +83,7 @@ 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)