Ensure a string return by curlExec

Fix error like

    TypeError: Return value of Sabre\HTTP\Client::curlExec() must be of
    the type string, boolean returned

lib/Client.php

@@ -588,7 +588,12 @@ protected function curlExec($curlHandle): string
     {
         $this->headerLinesMap[(int) $curlHandle] = [];
 
-        return curl_exec($curlHandle);
+        $result = curl_exec($curlHandle);
+        if (false === $result) {
+            $result = '';
+        }
+
+        return $result;
     }
 
     /**