test: speedup bip324_cipher.py unit test

Executing the unit tests for the bip324_cipher.py module currently
takes quite long (>60 seconds on my notebook). Most time here is spent
in empty plaintext/ciphertext encryption/decryption loops:

    ....
    for _ in range(msg_idx):
        enc_aead.encrypt(b"", b"")
    ...
    for _ in range(msg_idx):
        enc_aead.decrypt(b"", bytes(16))
    ...

Their sole purpose is increasing the FSChaCha20Poly1305 packet
counters in order to trigger rekeying, i.e. the actual
encryption/decryption is not relevant, as the result is thrown away.
This commit speeds up the tests by supporting to pass "None" as
plaintext/ciphertext, indicating to the routines that no actual
encryption/decryption should be done.

master branch:

$ python3 -m unittest ./test/functional/test_framework/crypto/bip324_cipher.py
..
----------------------------------------------------------------------
Ran 2 tests in 64.658s

PR branch:

$ python3 -m unittest ./test/functional/test_framework/crypto/bip324_cipher.py
..
----------------------------------------------------------------------
Ran 2 tests in 0.822s

test/functional/test_framework/crypto/bip324_cipher.py

@@ -25,6 +25,8 @@ def pad16(x):
 
 def aead_chacha20_poly1305_encrypt(key, nonce, aad, plaintext):
     """Encrypt a plaintext using ChaCha20Poly1305."""
+    if plaintext is None:
+        return None
     ret = bytearray()
     msg_len = len(plaintext)
     for i in range((msg_len + 63) // 64):
@@ -42,7 +44,7 @@ def aead_chacha20_poly1305_encrypt(key, nonce, aad, plaintext):
 
 def aead_chacha20_poly1305_decrypt(key, nonce, aad, ciphertext):
     """Decrypt a ChaCha20Poly1305 ciphertext."""
-    if len(ciphertext) < 16:
+    if ciphertext is None or len(ciphertext) < 16:
         return None
     msg_len = len(ciphertext) - 16
     poly1305 = Poly1305(chacha20_block(key, nonce, 0)[:32])
@@ -191,11 +193,11 @@ class TestFrameworkAEAD(unittest.TestCase):
             dec_aead = FSChaCha20Poly1305(key)
 
             for _ in range(msg_idx):
-                enc_aead.encrypt(b"", b"")
+                enc_aead.encrypt(b"", None)
             ciphertext = enc_aead.encrypt(aad, plain)
             self.assertEqual(hex_cipher, ciphertext.hex())
 
             for _ in range(msg_idx):
-                dec_aead.decrypt(b"", bytes(16))
+                dec_aead.decrypt(b"", None)
             plaintext = dec_aead.decrypt(aad, ciphertext)
             self.assertEqual(plain, plaintext)