Update HexFormatUtil.java

[inonwir]

We are working on a research project for improving open-source projects, by using the latest accepted answer from Stack Overflow that matched with your code snippet. We found this recommendation for improving your code from https://stackoverflow.com/questions/60266789.

Type of Recommendation: Add Null Check Reason: Prevent NullPointerException Type: Bug Influences: Fix method bytesToHexString

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