restrict use of os.environ in Configuration

Only use the OS environment, when the environ parameter is set
to None. Currently it would use the OS env on an empty dict.

src/nominatim_db/config.py

@@ -61,7 +61,7 @@ class Configuration:
 
     def __init__(self, project_dir: Optional[Union[Path, str]],
                  environ: Optional[Mapping[str, str]] = None) -> None:
-        self.environ = environ or os.environ
+        self.environ = os.environ if environ is None else environ
         self.config_dir = paths.CONFIG_DIR
         self._config = dotenv_values(str(self.config_dir / 'env.defaults'))
         if project_dir is not None: