Update HttpServer.java

[cragkhit]

We are researchers from Mahidol University, Thailand, and the State University of Ceará, Brazil, working on a research project for improving open-source projects by using the latest accepted answer from Stack Overflow that matched your code snippet. We found this recommendation for improving your code from https://stackoverflow.com/questions/59664132/spring-cloud-contract-testing-doesnt-work-for-jaxrsclient/59664498#59664498.

Note: Our study is approved by the Institutional Review Board of Mahidol University. You can find the participant information sheet explaining this study https://drive.google.com/file/d/1ml5AqrtWQ9pnifTQyTFTcWQmwp6RuPA7/view?usp=sharing.

Changes:

Adding a null check before closing the server.