[common] Keep blank variant object keys unshredded during schema inference

[zhuxiangyi]

Purpose

This is a bug fix. With variant.inferShreddingSchema=true, a Variant object whose key is empty or whitespace-only fails the whole file write:

CREATE TABLE T (id INT, v VARIANT) TBLPROPERTIES ('variant.inferShreddingSchema' = 'true');
INSERT INTO T VALUES (1, parse_json('{"":1,"a":2}'));
-- Job aborted ... java.lang.IllegalArgumentException: Field names must contain at least one non-whitespace character.
--   at org.apache.paimon.types.RowType.validateFields(RowType.java:283)
--   at org.apache.paimon.data.variant.InferVariantShreddingSchema.schemaOf(InferVariantShreddingSchema.java:318)

InferVariantShreddingSchema.schemaOf turns every key of a sampled object into a DataField and builds a RowType from them, and RowType rejects a blank field name. An empty key is a valid Variant object key (the same row writes and reads back fine with inference off), so inference must not turn it into a write failure. Other unusual keys (a.b, keys with spaces, typed_value / value / metadata, non-ASCII) already work.

The fix skips blank keys when building the inferred object type, so they stay in the unshredded value while the remaining keys are still shredded; an object made only of blank keys stays unshredded. The alphabetical-order check now iterates the kept fields rather than the original object size.

Tests

The new tests reproduce the bug: on master they fail with the IllegalArgumentException above, and pass with this change.

• InferVariantShreddingSchemaTest#testInferSchemaSkipsBlankKeys: {"":1," ":2,"a":3} infers to a typed object with only a; {"":1} alone stays unshredded.
• InferVariantShreddingWriteTest#testInferSchemaSkipsBlankKeys: writes such rows with inference on, checks the physical Parquet schema, the reconstructed Variant (blank keys preserved) and typed extraction of $.a.
• VariantTestBase: inserts {"":1,"a":2} and reads it back with to_json and variant_get; runs under all four Spark 4.x configurations. Verified on Spark 4.1.2: 116 tests pass.

API and Format

No.

Documentation

No.