offer: fix type to the getchaininfo rpc

This is violate what the docs of getchainfo is telling us that the
last_block_height is a u64.

```
2024-05-17T09:53:23.464Z **BROKEN** plugin-offers: Got error reply to getchaininfo: '{\"error\":{\"code\":-1,\"data\":null,\"message\":\"invalid type: string \\\"0\\\", expected u64\"},\"id\":\"init/offers:getchaininfo#1\",\"jsonrpc\":\"2.0\"}\n\n'
2024-05-17T09:53:23.496Z INFO    plugin-offers: Killing plugin: exited before replying to init
2024-05-17T09:53:23.496Z **BROKEN** plugin-offers: Plugin marked as important, shutting down lightningd!
2024-05-17T09:53:23.496Z DEBUG   lightningd: io_break: lightningd_exit
2024-05-17T09:53:23.496Z **BROKEN** plugin-topology: Reading JSON input: Connection reset by peer
2024-05-17T09:53:23.504Z INFO    plugin-topology: Killing plugin: exited before replying to init
2024-05-17T09:53:23.504Z **BROKEN** plugin-topology: Plugin marked as important, shutting down lightningd!
2024-05-17T09:53:23.504Z DEBUG   lightningd: io_break: lightningd_exit
2024-05-17T09:53:23.504Z DEBUG   plugin-bookkeeper: Setting up database at sqlite3://accounts.sqlite3
2024-05-17T09:53:23.504Z DEBUG   connectd: REPLY WIRE_CONNECTD_START_SHUTDOWN_REPLY with 0 fds
2024-05-17T09:53:23.504Z DEBUG   lightningd: io_break: connectd_start_shutdown_reply
```

Fixing the following crash

```
Got error reply to getchaininfo: '{"error":{"code":-1,"data":null,"message":"invalid type: string \"0\", expected u64"},"id":"init/offers:getchaininfo#1","jsonrpc":"2.0"}

'lightningd: lightningd already running? Error locking PID file: Resource temporarily unavailable
```

Fixes: 847208f
Changelog-None: offer: fix type to the getchaininfo rpc
Signed-off-by: Vincenzo Palazzo <[email protected]>

plugins/offers.c

@@ -1230,7 +1230,7 @@ static const char *init(struct plugin *p,
 		 "{id:%}", JSON_SCAN(json_to_pubkey, &id));
 
 	rpc_scan(p, "getchaininfo",
-		 take(json_out_obj(NULL, "last_height", "0")),
+		 take(json_out_obj(NULL, "last_height", NULL)),
 		 "{headercount:%}", JSON_SCAN(json_to_u32, &blockheight));
 	assert(blockheight);