Add not null Preconditions check to varargs constructor for BaseCombinedTask

[kbendick]

I was looking at another PR and checking for nullability concerns, when I noticed that there are two constructors for BaseCombineTask. There's one constructor that takes in a List<FileScanTask> and one that uses varargs FileScanTask.... The List constructor specifically checks for null, however, the varargs one does not.

However, because arrays are objects in java, the FileScanTask... when passed in null does not give new FileScanTask[]{null} or new FileScanTask[0], it simply passes null.

Therefore, it doesn't seem overly cautious to add a precondition check for null here since we have it already.

The code in question is here:
https://github.com/apache/iceberg/blob/master/core/src/main/java/org/apache/iceberg/BaseCombinedScanTask.java#L32-L39

I can submit a patch.

[kbendick]

Closing this issue as this has been fixed.