fix: tolerate NUL-bearing charset in part decode

a MIME part charset param containing a NUL character (e.g. malformed/adversarial
.eml input) makes codec lookup raise ValueError instead of the LookupError/TypeError
already handled here, escaping uncaught through extract_code. Catch ValueError too
and fall back to utf-8 like the existing bad-charset path, mirroring
decode_header_value's existing (UnicodeDecodeError, LookupError, ValueError) pattern.

Signed-off-by: disqualifier <dev@disqualifier.me>

src/aiomail/extract.py

@@ -57,7 +57,7 @@ def _decode_part(part: email.message.Message) -> Optional[str]:
     charset = part.get_content_charset() or "utf-8"
     try:
         return payload.decode(charset, errors="replace")
-    except (LookupError, TypeError):
+    except (LookupError, TypeError, ValueError):
         return payload.decode("utf-8", errors="replace")