Document empty response handling (204 No Content)

Add sections in apis.md and pull-and-push.md explaining how empty
upstream responses are handled in raw mode (signed null) vs encoded
mode (502 error).

Author: andreogle

book/docs/concepts/pull-and-push.md

@@ -61,6 +61,23 @@ Raw mode is useful when the consumer needs the full JSON structure or when multi
 needed. The signature still proves the data came from this airnode, but the data itself is not ABI-encoded for on-chain
 use.
 
+### Response (empty)
+
+When the upstream API returns an empty body (e.g. HTTP 204 No Content), the behavior depends on encoding:
+
+- **Raw endpoints** return `rawData: null` with a valid signature, attesting that the API returned no content.
+- **Encoded endpoints** return HTTP 502 because there is no data to extract and encode.
+
+```json
+{
+  "airnode": "0xd1e98F3Ac20DA5e4da874723517c914a31b0e857",
+  "endpointId": "0xa1b2...endpoint-id-hash",
+  "timestamp": 1711234567,
+  "rawData": null,
+  "signature": "0x1234...65-byte-ecdsa-signature"
+}
+```
+
 ## Push
 
 Push is continuous. Airnode calls APIs on a background interval, signs the results, and stores them in an in-memory

book/docs/config/apis.md

@@ -411,6 +411,18 @@ endpoints:
     # No encoding -- raw JSON response is signed
 ```
 
+### Empty responses (204 No Content)
+
+When the upstream API returns an empty body (e.g. HTTP 204), the behavior depends on the encoding mode:
+
+- **Raw mode** — returns `rawData: null` with a valid signature. The signature covers `keccak256(toHex("null"))`,
+  providing a verifiable attestation that the API returned no content.
+- **Encoded mode** — returns HTTP 502 with `"API returned no data to encode"`. There is no value to extract via
+  JSONPath, so encoding cannot proceed.
+
+Empty JSON objects (`{}`) behave similarly — raw mode signs them as-is, while encoded mode fails if the JSONPath finds no
+value at the configured path.
+
 ### Requester-specified encoding
 
 Clients can control encoding by passing reserved parameters in their request body: `_type`, `_path`, and optionally