Fix perfect match missed for headers with empty values

HeaderTable.search() returns (index, name, value) for perfect matches
and (index, name, None) for partial matches. The encoder distinguishes
them with `if perfect:`, but this fails when value is b"" because
empty bytes are falsy in Python.

46 of 61 static table entries have an empty value (e.g. :authority,
accept-charset, accept-language, age, allow, …). When encoding a
header that perfectly matches one of these entries, the encoder falls
through to the indexed-literal path — using 2+ bytes instead of 1 and
unnecessarily adding the entry to the dynamic table.

Change the check to `if perfect is not None:` so that b"" is treated
as a valid perfect match.

Author: bysiber

src/hpack/hpack.py

@@ -313,7 +313,7 @@ def add(self, to_add: tuple[bytes, bytes], sensitive: bool, huffman: bool = Fals
         # can use the indexed literal.
         index, name, perfect = match
 
-        if perfect:
+        if perfect is not None:
             # Indexed representation.
             encoded = self._encode_indexed(index)
         else: