fix(query): count_distinct needs to handle nullable correctly

[forsaken628]

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Summary

• While AggregateFunctionCombinatorNull previously excluded rows with null in any column,
  this PR separates count_distinct into a separate function that only excludes rows where all columns are null.
• Improvements to sqllogictests' checking of ColumnType.

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Explain why

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

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This change is 

[chatgpt-codex-connector]

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