[Java] fix java apache-httpclient set basePath

Created by: JoeCqupt

when i use java-sdk generated by openapi-generater (library:apache-httpclient).

below code is not work

apiClient.setBasePath("https://{host}:{port}");

the reason is method: ApiClient#buildUrl()

private String buildUrl(String path, List<Pair> queryParams, List<Pair> collectionQueryParams) {
    String baseURL;
    if (serverIndex != null) {
      if (serverIndex < 0 || serverIndex >= servers.size()) {
        throw new ArrayIndexOutOfBoundsException(String.format(
          "Invalid index %d when selecting the host settings. Must be less than %d", serverIndex, servers.size()
        ));
      }
      baseURL = servers.get(serverIndex).URL(serverVariables);
    } else {
      baseURL = basePath;
    }
....

the variable serverIndex default is 0. so the baseUrl always is the serverIndex of servers

if we don't serverIndex = null . setBasePath() will not work.

apiClient.setServerIndex(null);

i think we should set serverIndex default is null.

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