Default global state variables to the correct type

Code gets easier to understand if variables don't change their type.

benchmark/space/global_state.py

@@ -3,16 +3,16 @@ import threading
 
 class G:
     """Global state of the Benchmark Client."""
-    result_platform = None
-    progress_status = None
-    result_stats = None
-    result_dict = None
+    result_platform = ''
+    progress_status = ''
+    result_stats = ''
+    result_dict = {}
     background_image_path = ""
     scene_status = {}
     cancel = False
 
-    cached_system_info = None
-    cached_compute_devices = None
+    cached_system_info = {}
+    cached_compute_devices = []
     results_submitted = False
 
     images = {}
@@ -22,10 +22,10 @@ class G:
     @classmethod
     def reset(cls):
         """Reset the global state."""
-        cls.result_platform = None
-        cls.progress_status = None
-        cls.result_stats = None
-        cls.result_dict = None
+        cls.result_platform = ''
+        cls.progress_status = ''
+        cls.result_stats = ''
+        cls.result_dict = {}
         cls.background_image_path = ""
         cls.scene_status = {}
         cls.results_submitted = False