fix NPE that can happen when merging all null nested v4 format columns #14068
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Description
Fixes another regression related to #14014, this time when processing all null or nonexistent columns with the 'json' (v4) format. Added a test to ensure that non-existent data is processed correctly, and tests for explicit 'auto' dimension schema with non-existent columns to ensure new common format also doesn't have any issues with non-existent columns.
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