NullPointerException when using Snowflake Catalog as source and Delta Lake as target

### Search before asking

- [x] I had searched in the [issues](https://github.com/apache/incubator-xtable/issues?q=is%3Aissue) and found no similar issues.

### Please describe the bug 🐞

I've followed the steps within this [blog](https://medium.com/@sagarlakshmipathy/how-to-interoperate-with-snowflake-and-open-table-formats-e70f453787a2) to generate Delta Lake metadata from a Snowflake managed Iceberg table but run into the following NullPointerException:
java.lang.NullPointerException: null
at org.apache.xtable.iceberg.IcebergColumnStatsConverter.lambda$fromIceberg$2(IcebergColumnStatsConverter.java:106) ~[xtable-utilities_2.12-0.2.0-SNAPSHOT-bundled.jar:0.2.0-SNAPSHOT]
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:195) ~[?:?]
at java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:177) ~[?:?]
at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1655) ~[?:?]
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:484) ~[?:?]
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:474) ~[?:?]
at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:913) ~[?:?]
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) ~[?:?]
at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:578) ~[?:?]
at org.apache.xtable.iceberg.IcebergColumnStatsConverter.fromIceberg(IcebergColumnStatsConverter.java:119) ~[xtable-utilities_2.12-0.2.0-SNAPSHOT-bundled.jar:0.2.0-SNAPSHOT]
at org.apache.xtable.iceberg.IcebergDataFileExtractor.fromIceberg(IcebergDataFileExtractor.java:64) ~[xtable-utilities_2.12-0.2.0-SNAPSHOT-bundled.jar:0.2.0-SNAPSHOT]
at org.apache.xtable.iceberg.IcebergDataFileExtractor.fromIceberg(IcebergDataFileExtractor.java:53) ~[xtable-utilities_2.12-0.2.0-SNAPSHOT-bundled.jar:0.2.0-SNAPSHOT]
at org.apache.xtable.iceberg.IcebergConversionSource.fromIceberg(IcebergConversionSource.java:175) ~[xtable-utilities_2.12-0.2.0-SNAPSHOT-bundled.jar:0.2.0-SNAPSHOT]
at org.apache.xtable.iceberg.IcebergConversionSource.getCurrentSnapshot(IcebergConversionSource.java:156) ~[xtable-utilities_2.12-0.2.0-SNAPSHOT-bundled.jar:0.2.0-SNAPSHOT]
at org.apache.xtable.spi.extractor.ExtractFromSource.extractSnapshot(ExtractFromSource.java:38) ~[xtable-utilities_2.12-0.2.0-SNAPSHOT-bundled.jar:0.2.0-SNAPSHOT]
at org.apache.xtable.conversion.ConversionController.syncSnapshot(ConversionController.java:282) ~[xtable-utilities_2.12-0.2.0-SNAPSHOT-bundled.jar:0.2.0-SNAPSHOT]
at org.apache.xtable.conversion.ConversionController.syncTableFormats(ConversionController.java:202) ~[xtable-utilities_2.12-0.2.0-SNAPSHOT-bundled.jar:0.2.0-SNAPSHOT]
at org.apache.xtable.conversion.ConversionController.sync(ConversionController.java:105) ~[xtable-utilities_2.12-0.2.0-SNAPSHOT-bundled.jar:0.2.0-SNAPSHOT]
at org.apache.xtable.utilities.RunSync.main(RunSync.java:191) [xtable-utilities_2.12-0.2.0-SNAPSHOT-bundled.jar:0.2.0-SNAPSHOT]

### Are you willing to submit PR?

- [ ] I am willing to submit a PR!
- [ ] I am willing to submit a PR but need help getting started!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

Reproduce the Snowflake-managed Iceberg to Delta Lake conversion described in the linked blog, then inspect IcebergColumnStatsConverter.fromIceberg at line 106 and its callers in IcebergDataFileExtractor and IcebergConversionSource. Done means the conversion completes without the reported NullPointerException and generates the Delta Lake metadata.

Written by the indexing model from the issue text.