Delete the U2F counter if it's set to None.

src/apps/common/storage.py

@@ -45,6 +45,10 @@ def _get_bool(app: int, key: int, public: bool = False) -> bool:
 
 
 def _set_counter(app: int, key: int, count: int, public: bool = False) -> None:
+    if count is None:
+        config.delete(app, key, public)
+        return
+
     value = count.to_bytes(_COUNTER_HEAD_LEN, "big")
     if public:
         value += _COUNTER_TAIL_LEN * b"\xff"

tests/test_apps.common.storage.py

@@ -15,6 +15,10 @@ class TestConfig(unittest.TestCase):
         storage.set_u2f_counter(350)
         for i in range(351, 500):
             self.assertEqual(storage.next_u2f_counter(), i)
+        storage.set_u2f_counter(0)
+        self.assertEqual(storage.next_u2f_counter(), 1)
+        storage.set_u2f_counter(None)
+        self.assertEqual(storage.next_u2f_counter(), 0)
 
 
 if __name__ == '__main__':