Fix backward incompatible config behavior (#4408)

nni/experiment/config/exp_config.py

@@ -89,7 +89,7 @@ class ExperimentConfig(ConfigBase):
             for algo_type in ['tuner', 'assessor', 'advisor']:
                 # add placeholder items, so users can write `config.tuner.name = 'random'`
                 if getattr(self, algo_type) is None:
-                    setattr(self, algo_type, _AlgorithmConfig(name='_none_'))
+                    setattr(self, algo_type, _AlgorithmConfig(name='_none_', class_args={}))
         elif not utils.is_missing(self.training_service):
             # training service is set via json or constructor
             if isinstance(self.training_service, list):

test/ut/experiment/test_exp_config.py

+import copy
 import os.path
 from pathlib import Path
 
@@ -46,6 +47,9 @@ minimal_canon = {
     },
 }
 
+minimal_canon_2 = copy.deepcopy(minimal_canon)
+minimal_canon_2['tuner']['classArgs'] = {}
+
 ## detailed config ##
 
 detailed_canon = {
@@ -100,7 +104,7 @@ def test_all():
     minimal = ExperimentConfig(**minimal_json)
     assert minimal.json() == minimal_canon
 
-    assert minimal_class.json() == minimal_canon
+    assert minimal_class.json() == minimal_canon_2
 
     detailed = ExperimentConfig.load(expand_path('assets/config.yaml'))
     assert detailed.json() == detailed_canon