books: Loader.load_none() returns default options_map.

This way it's still safe to assume all the expected keys are there.

conservancy_beancount/books.py

@@ -18,7 +18,8 @@ import datetime
 
 from pathlib import Path
 
-from beancount import loader as bc_loader
+import beancount.loader as bc_loader
+import beancount.parser.options as bc_options
 
 from typing import (
     Any,
@@ -204,4 +205,4 @@ class Loader:
             'lineno': lineno,
         }
         errors: Errors = [Error(source, "no books to load in configuration", None)]
-        return [], errors, {}
+        return [], errors, bc_options.OPTIONS_DEFAULTS.copy()