Core: Fix overflow due to default value on timestamp nanos

[ebyhr]

Iceberg throws exceptions if JSON contains default values on timestamp nanos:

Caused by: java.lang.ArithmeticException: long overflow
	at java.base/java.lang.Math.multiplyExact(Math.java:1035)
	at org.apache.iceberg.util.DateTimeUtil.microsToNanos(DateTimeUtil.java:101)
	at org.apache.iceberg.expressions.Literals$TimestampLiteral.to(Literals.java:447)
	at org.apache.iceberg.expressions.Literals$LongLiteral.to(Literals.java:306)
	at org.apache.iceberg.types.Types$NestedField.castDefault(Types.java:890)
	at org.apache.iceberg.types.Types$NestedField.<init>(Types.java:882)
	at org.apache.iceberg.types.Types$NestedField$Builder.build(Types.java:850)
	at org.apache.iceberg.SchemaParser.structFromJson(SchemaParser.java:242)
	at org.apache.iceberg.SchemaParser.typeFromJson(SchemaParser.java:186)
	at org.apache.iceberg.SchemaParser.fromJson(SchemaParser.java:277)
	at org.apache.iceberg.util.JsonUtil.parse(JsonUtil.java:104)
	at org.apache.iceberg.SchemaParser.lambda$fromJson$1(SchemaParser.java:296)
	at com.github.benmanes.caffeine.cache.BoundedLocalCache.lambda$doComputeIfAbsent$14(BoundedLocalCache.java:2704)
	at java.base/java.util.concurrent.ConcurrentHashMap.compute(ConcurrentHashMap.java:1932)
	at com.github.benmanes.caffeine.cache.BoundedLocalCache.doComputeIfAbsent(BoundedLocalCache.java:2702)
	at com.github.benmanes.caffeine.cache.BoundedLocalCache.computeIfAbsent(BoundedLocalCache.java:2684)
	at com.github.benmanes.caffeine.cache.LocalCache.computeIfAbsent(LocalCache.java:112)
	at com.github.benmanes.caffeine.cache.LocalManualCache.get(LocalManualCache.java:63)
	at org.apache.iceberg.SchemaParser.fromJson(SchemaParser.java:296)

[manuzhang]

can you please add some comments here?