fix: handle empty api responses more gracefully

src/BusProNet/ApiClient.php

@@ -688,6 +688,14 @@ class ApiClient
         // message length (10 bytes) is prepended to actual message
         $xml = substr($response, 10);
 
+        if ('' === $xml) {
+            $this->logger->error('Empty response body received from API (header-only response)', [
+                'request_id' => $requestId,
+                'raw_length' => strlen($response),
+            ]);
+            throw new ApiClientException('Empty response body received from API');
+        }
+
         if (true === $debug || true === $this->config['debug']) {
             $this->dumpXmlToFile('response', $requestId, $xml);
         }
@@ -898,6 +906,15 @@ class ApiClient
             $response .= $chunk;
         }
 
+        if (0 === strlen($response)) {
+            $elapsedTime = microtime(true) - $this->operationStartTime;
+            $this->logger->warning('Server closed connection without sending data', [
+                'elapsed_time' => $elapsedTime,
+                'read_attempts' => $readAttempts,
+            ]);
+            throw new ImmediateConnectionCloseException('Server closed connection without sending data');
+        }
+
         return $response;
     }