[jvm-packages] handle null to avoid potential NullPointerException

[ShangjinTang]

This PR add null checks for jvm-packages/xgboost4j/src/main/java/ml/dmlc/xgboost4j/java/TrackerProperties.java.

It retrieves the current thread's class loader using Thread.currentThread().getContextClassLoader() and verifies if it is not null.

[trivialfis]

Hi, thank you for the PR. Could you please elaborate on under what circumstances it would be null, and what happens if it's null after this PR?

[ShangjinTang]

Hello,

This is a failure that we scanned from our static code check tool and marked as "must be fixed".
I also searched from web, some developers have occured that Thread.currentThread().getContextClassLoader() might be null (although I cannot reproduce the issue).
I am using the xgboost4j library on Android, so the entire process would crash if contextClassLoader is null.

Thanks

[wbo4958]

Looks like TrackerProperties.java is not used anymore after tracker reworking.