[Lorenz] Handle graphs without metadata

Metadata was added recently, but older graphs do not contain it. Avoid
errors when reading these graphs.

Change-Id: Ib5c6eb01d53a1e67a0fb5b7451875e891fe3184d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2522240Reviewed-by: Mohamed Heikal <mheikal@chromium.org>
Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826507}

tools/android/dependency_analysis/modularization_stats.py

@@ -19,6 +19,8 @@ CLASSES_TO_COUNT_INBOUND = ['ChromeActivity', 'ChromeTabbedActivity']
 
 
 def _copy_metadata(metadata: Dict) -> Dict[str, str]:
+    if metadata is None:
+        return {}
     return {f'meta_{key}': value for key, value in metadata.items()}
 
 

tools/android/dependency_analysis/serialization.py

@@ -143,7 +143,7 @@ def load_class_graph_from_file(
         json_obj = json.load(json_file)
         class_graph_json_obj = json_obj[json_consts.CLASS_GRAPH]
         return create_class_graph_from_json_obj(
-            class_graph_json_obj), json_obj[json_consts.BUILD_METADATA]
+            class_graph_json_obj), json_obj.get(json_consts.BUILD_METADATA)
 
 
 def load_class_and_package_graphs_from_file(