Fix reading map value contains variant null value

[chenjian2664]

Previously, a null value was appended when the VARIANT block had zero positions. This caused a mismatch between key and value block sizes, leading to errors when building map types.

Description

When reading the null map value and the value type is a variant type, we will encounter:

Caused by: java.lang.IllegalArgumentException: keyBlock and valueBlock has different size: 0 1
	at io.trino.spi.block.MapBlock.validateConstructorArguments(MapBlock.java:143)
	at io.trino.spi.block.MapBlock.fromKeyValueBlock(MapBlock.java:82)
	at io.trino.spi.block.MapBlock.fromKeyValueBlock(MapBlock.java:71)
	at io.trino.spi.type.MapType.createBlockFromKeyValue(MapType.java:424)
	at io.trino.parquet.reader.ParquetReader.readMap(ParquetReader.java:571)
	at io.trino.parquet.reader.ParquetReader.readColumnChunk(ParquetReader.java:755)
	at io.trino.parquet.reader.ParquetReader.readBlock(ParquetReader.java:741)
	at io.trino.parquet.reader.ParquetReader$ParquetSourcePage.getBlock(ParquetReader.java:353)

Additional context and related issues

Release notes

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(x) Release notes are required, with the following suggested text:

## Delta Lake
* Fix reading `null` map with `VARIANT` as value type. ({issue}`issuenumber`)

[Copilot]

Pull Request Overview

This PR fixes a bug where reading null map values with VARIANT as the value type would fail due to a mismatch between key and value block sizes. The fix removes problematic null value appending in the VARIANT reader when there are zero positions.

• Removes null appending logic in VARIANT reader that caused block size mismatches
• Updates test data to include map columns with VARIANT values for better coverage
• Expands test cases to verify the fix handles null maps with VARIANT values correctly

Reviewed Changes

Copilot reviewed 5 out of 7 changed files in this pull request and generated 1 comment.

File	Description
ParquetReader.java	Removes problematic null appending logic in readVariant method
TestDeltaLakeBasic.java	Adds comprehensive test coverage for null maps with VARIANT values
test_variant_null/_delta_log/*.json	Updates test metadata to include map columns with VARIANT values
test_variant_null/README.md	Documents updated test data structure with map column

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[ebyhr]

The indentation at L1647 is wrong.

[ebyhr]

Fix reading null map with VARIANT as value type. ({issue}issuenumber)

@chenjian2664 When fixing a query failure, please start the sentence with "Fix failure …". There are some common patterns for release note entries. Also, replace issuenumber with the PR number when there is no corresponding issue link.