[flink] Fix decimal column analysis with null statistics

[ArnavBalyan]

Purpose

• Analyzing a table causes query failure when a decimal column contains only null values.
• Flink reports null min/max, but read attempts a conversion to decimal, causing NPE.
• Preserve the null min/max values and convert the remaining available decimal statistics.

Tests

• UT

[ArnavBalyan]

cc @JingsongLi thank you