Fix perfect match missed for headers with empty values

[bysiber]

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 check fails when value is b"" because empty bytes are falsy in Python.

46 of 61 static table entries have an empty value (:authority, accept-charset, accept-language, age, allow, authorization, cache-control, etc.). When encoding a header that perfectly matches one of those entries, the encoder falls through to the indexed-literal branch instead:

>>> from hpack import Encoder
>>> e = Encoder()
>>> e.encode([(b':authority', b'')]).hex()
'4180'   # indexed literal, 2 bytes — also adds to dynamic table

Expected: 81 (indexed representation, 1 byte, no dynamic table entry).

The indexed-literal path also calls self.header_table.add(name, value), which adds an unnecessary duplicate of the static entry to the dynamic table, wasting space and evicting useful entries.

This changes if perfect: to if perfect is not None:.