Transport parameters with zero-length integer values accepted (RFC 9000 §18.2)

quiche accepts QUIC transport parameters with zero-length values for integer fields (e.g. `max_idle_timeout`, `max_ack_delay`). The empty value buffer gets interpreted as varint 0 via the FFI bridge.

RFC 9000 §18.2 requires integer transport parameters to carry a varint value. A zero-byte encoding is not a valid varint.

**Example:** `0100` (max_idle_timeout with length 0) — quiche treats this as max_idle_timeout=0.

ngtcp2 and lsquic both reject with TRANSPORT_PARAMETER_ERROR.

**Comparison:**
| Seed | ngtcp2 | lsquic | quiche |
|------|--------|--------|--------|
| max_idle_timeout empty `0100` | REJECT | REJECT | **ACCEPT** |
| max_ack_delay empty `0b00` | REJECT | REJECT | **ACCEPT** |
| Legal zero `010100` | ACCEPT | ACCEPT | ACCEPT |

Root cause is in `quiche/src/transport_params.rs` — `val.get_varint()` on an empty value buffer is treated as 0 instead of an error.

**Fix:** Reject integer TPs when decoded value length is 0 before calling varint parse.

Found through differential fuzzing (PathDiff v23), also filed with picoquic.

Start in quiche/src/transport_params.rs at the val.get_varint() call used for integer transport parameters. Check the decoding path for the empty values in max_idle_timeout and max_ack_delay, then verify that those encodings are rejected while the legal zero encoding 010100 remains accepted.

Written by the indexing model from the issue text.