Fix typing errors

Author: Legrandin

lib/Crypto/Protocol/HPKE.py

@@ -94,7 +94,7 @@ def __init__(self,
                  aead_id: AEAD,
                  mode: MODE):
 
-        self.enc: bytes = enc
+        self.enc: bytes = b'' if enc is None else enc
         """The encapsulated session key."""
 
         self._verify_psk_inputs(mode, psk_pair)

lib/Crypto/SelfTest/Protocol/test_HPKE.py

@@ -335,7 +335,7 @@ class HPKE_TestVectors(unittest.TestCase):
     def setUp(self):
         self.vectors = []
         try:
-            import pycryptodome_test_vectors
+            import pycryptodome_test_vectors    # type: ignore
             init_dir = os.path.dirname(pycryptodome_test_vectors.__file__)
             full_file_name = os.path.join(init_dir, "Protocol", "HPKE-test-vectors.json")
             with open(full_file_name, "r") as f: