hkdf: Remove duplicate check if the salt is None

The second if isn't needed since we initialize the salt with zeroes
earlier. If instead we meant to pass an empty bytes class to the HMAC
implementation, the end result would be the same, since it's gonna get
padded with zeroes in the HMAC code.

RNS/Cryptography/HKDF.py

@@ -39,9 +39,6 @@ def hkdf(length=None, derive_from=None, salt=None, context=None):
     if salt == None or len(salt) == 0:
         salt = bytes([0] * hash_len)
 
-    if salt == None:
-        salt = b""
-
     if context == None:
         context = b""