Update HexConvert.java

[cragkhit]

Updating the bytesToHexString() method to avoid NullPointerException.

Description

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/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.

Fixes Issue #1178.

Type of change

• Bug fix.
• New feature.
• Enhancement.
• Unit test.
• Breaking change (a fix or feature that causes existing functionality to not work as expected).
• Requires documentation update.

Testing