docs: private String getNegotiateToken() throws GSSException {
          if (spnegoNegotiator == null) {
              throw new IllegalStateException("Negotiator not yet initialized.");
          }
          // Perform GSS Dance
          String authString;
          if (StringUtils.hasText(challenge)) {
              authString = spnegoNegotiator.send(challenge);
          } else {
              authString = spnegoNegotiator.send();
          }
          this.challenge = null;
  
          // Prepend the authentication scheme to use
          if (authString != null) {
              authString = EsHadoopAuthPolicies.NEGOTIATE + " " + authString;
          }
          return authString;
      }