fix(go): scan nullable type/payload in receive

[NikolayS]

Bug

pgque.message.type and .payload are nullable text (sql/pgque-api/receive.sql), and PgQ legally stores NULL ev_type/ev_data (direct pgque.insert_event calls, trigger-based producers). The Go client declared Message.Type and Message.Payload as non-nullable string and scanned the columns directly into them in both Receive and ReceiveCoop.

Scanning a NULL into Go string errors (cannot scan NULL into *string), which fails the WHOLE batch: Receive returns an error, the batch is never acked, and next_batch returns the same batch on every poll — a poison-message livelock from a single NULL event.

Fix

Both scan loops now go through a shared scanMessage helper that scans type/payload into nullable *string temporaries and maps NULL to "".

Approach rationale (over switching the fields to *string like Extra1..4):

• The public API is firmly built around plain string: consumer handler dispatch is keyed by msg.Type (map lookup, string concat in nack reasons), and the documented payload pattern is json.Unmarshal([]byte(msg.Payload), ...). Pointer fields would break every caller for a rare edge case.
• A NULL type maps to "", which has no registered handler, so the Consumer routes it through the existing unknown-handler policy path (Nack by default, or skip with AckUnknown) — no crash, no consumer.go changes needed.
• Server-side pgque.nack re-queries the canonical event row by msg_id from the active batch, so the client-side NULL -> "" mapping cannot corrupt retry/DLQ data.

The mapping is documented on the Message struct and on scanMessage.

Verification

TDD: the two new integration tests (clients/go/null_event_test.go) were written first and confirmed red on unfixed code, failing exactly on the bug:

$ PGQUE_TEST_DSN="postgresql:///pgque_gonull?host=/var/run/postgresql" go test -run 'NullTypeAndPayload' -v .
--- FAIL: TestReceive_NullTypeAndPayload
    receive with NULL type/payload: pgque: scan message: can't scan into dest[2] (col: type): cannot scan NULL into *string
--- FAIL: TestReceiveCoop_NullTypeAndPayload
    receive_coop with NULL type/payload: pgque: scan message: can't scan into dest[2] (col: type): cannot scan NULL into *string

The tests install an event with NULL type/data via select pgque.insert_event(queue, null, null), tick, then assert Receive/ReceiveCoop succeed, return the message with Type == "" and Payload == "", and the batch is ackable.

After the fix (scratch DB pgque_gonull, fresh install of sql/pgque.sql on local PostgreSQL 16):

$ cd clients/go
$ PGQUE_TEST_DSN="postgresql:///pgque_gonull?host=/var/run/postgresql" go test -run 'NullTypeAndPayload' -v .
--- PASS: TestReceive_NullTypeAndPayload (0.06s)
--- PASS: TestReceiveCoop_NullTypeAndPayload (0.07s)

$ PGQUE_TEST_DSN="postgresql:///pgque_gonull?host=/var/run/postgresql" go test ./...
ok  	github.com/NikolayS/pgque-go	16.089s

$ go vet ./...
(clean)

Addresses finding B4 of #283

https://claude.ai/code/session_01KAaEGkQZmey1D1xCsVGmqv

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