Core: Fix NPE of generateRandomMetrics in FileGenerationUtil

[dramaticlly]

fix NPE because NaturalOrderComparator.compare will not accept the null. And if the metrics mode is None or Count, today it returns null in FileGenerationUtil:: generateBound

iceberg/core/src/test/java/org/apache/iceberg/FileGenerationUtil.java

Lines 298 to 299 in 32b2f00

	if (mode instanceof None || mode instanceof Counts) {
	return null;

Cannot invoke "java.lang.Comparable.compareTo(Object)" because "c1" is null
java.lang.NullPointerException: Cannot invoke "java.lang.Comparable.compareTo(Object)" because "c1" is null
	at java.base/java.util.Comparators$NaturalOrderComparator.compare(Comparators.java:52)
	at java.base/java.util.Comparators$NaturalOrderComparator.compare(Comparators.java:47)
	at org.apache.iceberg.FileGenerationUtil.generateBounds(FileGenerationUtil.java:290)
	at org.apache.iceberg.FileGenerationUtil.generateRandomMetrics(FileGenerationUtil.java:193)
	at org.apache.iceberg.TestFileGenerationUtil.testBoundsForAllMetricsModes(TestFileGenerationUtil.java:113)

[dramaticlly]

FYI @aokolnychyi @nastra

[huaxingao]

LGTM

[singhpk234]

LGTM with a minor suggestions, thanks @dramaticlly