[SPARK-24742] Fix NullPointerexception in Field Metadata

## What changes were proposed in this pull request?

This pull request provides a fix for SPARK-24742: SQL Field MetaData was throwing an Exception in the hashCode method when a "null" Metadata was added via "putNull"

## How was this patch tested?

A new unittest is provided in org/apache/spark/sql/types/MetadataSuite.scala

Author: Kaya Kupferschmidt <[email protected]>

Closes #21722 from kupferk/SPARK-24742.

Author: kupferk