Always include partition column when get-table-layout

[ZhitongYan]

Issue #, if available:
When "PartitionCols" is null or empty for get-query-splits requests, following exception will be thrown:

java.lang.NullPointerException: Cannot invoke "org.apache.arrow.vector.FieldVector.getMinorType()" because "vector" is null
	at com.amazonaws.athena.connector.lambda.data.BlockUtils.setValue(BlockUtils.java:408)
	at com.amazonaws.athena.connector.lambda.data.Block.setValue(Block.java:149)
	at com.amazonaws.athena.connectors.snowflake.SnowflakeMetadataHandler.lambda$handleDirectQueryPartitions$3(SnowflakeMetadataHandler.java:317)
	at com.amazonaws.athena.connector.lambda.data.SimpleBlockWriter.writeRows(SimpleBlockWriter.java:56)
	at com.amazonaws.athena.connectors.snowflake.SnowflakeMetadataHandler.handleDirectQueryPartitions(SnowflakeMetadataHandler.java:315)
	at com.amazonaws.athena.connectors.snowflake.SnowflakeMetadataHandler.getPartitions(SnowflakeMetadataHandler.java:263)
	at com.amazonaws.athena.connector.lambda.handlers.MetadataHandler.doGetTableLayout(MetadataHandler.java:480)

Description of changes:
Above issue happens when BlockWriter does not have a partition schema. Hence adding the schema to avoid empty partition information for BlockWriter.

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[amazon-inspector-n-virginia]

⏳ I'm reviewing this pull request for security vulnerabilities and code quality issues. I'll provide an update when I'm done

[amazon-inspector-n-virginia]

⏳ I'm reviewing this pull request for security vulnerabilities and code quality issues. I'll provide an update when I'm done

[amazon-inspector-n-virginia]

✅ I finished the code review, and didn't find any security or code quality issues.

[amazon-inspector-n-virginia]

✅ I finished the code review, and didn't find any security or code quality issues.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.09%. Comparing base (fe0a5d9) to head (a43fa79).
⚠️ Report is 80 commits behind head on master.

Files with missing lines	Patch %	Lines
...connectors/snowflake/SnowflakeMetadataHandler.java	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3045      +/-   ##
============================================
+ Coverage     63.67%   66.09%   +2.41%     
- Complexity     4344     4763     +419     
============================================
  Files           621      635      +14     
  Lines         23286    24123     +837     
  Branches       2859     2995     +136     
============================================
+ Hits          14827    15943    +1116     
+ Misses         7070     6724     -346     
- Partials       1389     1456      +67     

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