Make inputs and backup_dir optional in the config file

They can come from -i and -o; missing ones were rejected as a bad configuration file (exit code 6).

simple_backup/simple_backup.py

@@ -946,19 +946,22 @@ def _read_config(config_file: str, user: str | None = None) -> ConfigArgs:
     config = configparser.ConfigParser(interpolation=None)
     config.read(config_file)
 
-    section = 'backup'
-
     # Allow compatibility with previous version of config file
-    try:
-        inputs = config.get(section, 'inputs')
-    except configparser.NoSectionError:
+    if config.has_section('backup'):
+        section = 'backup'
+    elif config.has_section('default'):
         section = 'default'
-        inputs = config.get(section, 'inputs')
+    else:
+        raise configparser.NoSectionError('backup')
 
-    config_args['inputs'] = list(set(_expand_inputs([i.strip() for i in inputs.split(',')], user)))
+    # inputs and backup_dir are optional, since -i and -o can give them. Missing ones are reported by check_params()
+    inputs = config.get(section, 'inputs', fallback=None)
+
+    if inputs is not None:
+        config_args['inputs'] = list(set(_expand_inputs([i.strip() for i in inputs.split(',')], user)))
 
     # '~' is expanded later, since for remote backups it refers to the home on the server
-    config_args['output'] = config.get(section, 'backup_dir')
+    config_args['output'] = config.get(section, 'backup_dir', fallback=None)
 
     exclude = config.get(section, 'exclude', fallback='')
     config_args['exclude'] = [e.strip() for e in exclude.split(',') if e.strip() != '']