use bytes() instead of b'' - avoid markdown issue

Currently github markdown renders `b''` inside `<source>` tags incorrectly. This makes `h = b''` show as `h = b` and creates some confusion.
The issue can be avoided by using bytes() to create empty byte array

bip-taproot.mediawiki

@@ -224,7 +224,7 @@ def taproot_output_script(internal_pubkey, script_tree):
      - a list of two elements, each with the same structure as script_tree itself"""
      - None
     if script_tree is None:
-        h = b''
+        h = bytes()
     else:
         _, h = taproot_tree_helper(script_tree)
     output_pubkey, _ = taproot_tweak_pubkey(internal_pubkey, h)