fix unit test

Author: themiswang

firebase-crashlytics/src/androidTest/java/com/google/firebase/crashlytics/internal/metadata/MetaDataStoreTest.java

@@ -150,6 +150,34 @@ public void testUpdateSessionId_notPersistUserIdToNewSessionIfNoUserIdSet() {
         .isFalse();
   }
 
+  @Test
+  public void testUpdateSessionId_notPersistCustomKeysToNewSessionIfNoCustomKeysSet() {
+    UserMetadata userMetadata = new UserMetadata(SESSION_ID_1, fileStore, worker);
+    userMetadata.setNewSession(SESSION_ID_2);
+    assertThat(fileStore.getSessionFile(SESSION_ID_2, UserMetadata.KEYDATA_FILENAME).exists())
+        .isFalse();
+  }
+
+  @Test
+  public void testUpdateSessionId_persistCustomKeysToNewSessionIfCustomKeysSet() {
+    UserMetadata userMetadata = new UserMetadata(SESSION_ID_1, fileStore, worker);
+    final Map<String, String> keys =
+        new HashMap<String, String>() {
+          {
+            put(KEY_1, VALUE_1);
+            put(KEY_2, VALUE_2);
+            put(KEY_3, VALUE_3);
+          }
+        };
+    userMetadata.setCustomKeys(keys);
+    userMetadata.setNewSession(SESSION_ID_2);
+    assertThat(fileStore.getSessionFile(SESSION_ID_2, UserMetadata.KEYDATA_FILENAME).exists())
+        .isTrue();
+
+    MetaDataStore metaDataStore = new MetaDataStore(fileStore);
+    assertThat(metaDataStore.readKeyData(SESSION_ID_2)).isEqualTo(keys);
+  }
+
   @Test
   public void testUpdateSessionId_persistUserIdToNewSessionIfUserIdSet() {
     String userId = "ThemisWang";

firebase-crashlytics/src/main/java/com/google/firebase/crashlytics/internal/metadata/UserMetadata.java

@@ -87,8 +87,12 @@ public void setNewSession(String sessionId) {
     synchronized (sessionIdentifier) {
       sessionIdentifier = sessionId;
       Map<String, String> keyData = customKeys.getKeys();
-      metaDataStore.writeUserData(sessionIdentifier, getUserId());
-      metaDataStore.writeKeyData(sessionIdentifier, keyData);
+      if (getUserId() != null) {
+        metaDataStore.writeUserData(sessionId, getUserId());
+      }
+      if (!keyData.isEmpty()) {
+        metaDataStore.writeKeyData(sessionId, keyData);
+      }
       // TODO(themis): adding feature rollouts later
     }
   }