Fix handling of empty responses.

src/githubctl/github_endpoint_watcher.rs

@@ -124,11 +124,11 @@ impl GithubEndpointWatcher {
 
         self.update_headers_from_response(&response, request_started_at)?;
         let response_status = response.status();
-        let body = response.json::<serde_json::Value>().await?;
 
         if let reqwest::StatusCode::NOT_MODIFIED = response_status {
             return Ok(None);
         } else {
+            let body = response.json::<serde_json::Value>().await?;
             return Ok(Some(body));
         }
     }