Merge pull request #1902 from Ayrx/make-check-clearer

Rewrite check to read clearer.
diff --git a/src/cryptography/hazmat/primitives/kdf/hkdf.py b/src/cryptography/hazmat/primitives/kdf/hkdf.py
index 65b7091..f738bbd 100644
--- a/src/cryptography/hazmat/primitives/kdf/hkdf.py
+++ b/src/cryptography/hazmat/primitives/kdf/hkdf.py
@@ -26,7 +26,7 @@
 
         self._algorithm = algorithm
 
-        if not isinstance(salt, bytes) and salt is not None:
+        if not (salt is None or isinstance(salt, bytes)):
             raise TypeError("salt must be bytes.")
 
         if salt is None:
@@ -77,7 +77,7 @@
 
         self._length = length
 
-        if not isinstance(info, bytes) and info is not None:
+        if not (info is None or isinstance(info, bytes)):
             raise TypeError("info must be bytes.")
 
         if info is None: