Add check for empty schema in parquet::schema::types::from_thrift_helper

[etseidl]

Which issue does this PR close?

Closes #6988.

Rationale for this change

Reading a file with an empty schema will fail in parquet::schema::types::from_thrift_helper because the root node in the schema is mistaken for a leaf node.

What changes are included in this PR?

Adds a check in from_thrift_helper for a root node with no children, and exits early if one is detected.

Are there any user-facing changes?

No

[etseidl]

This is @samgqroberts's reproducer from #6988

[alamb]

Thank you @etseidl and @samgqrobert 🙏

I have some suggestions on improving the tests, but I also think we can do that as a follow on PR (or never)

[alamb]

Very minor: you can use Schema::empty as well:

Suggested change

	let empty_fields: Vec<Field> = vec![];
	let empty_schema = Arc::new(Schema::new(empty_fields));
	let empty_schema = Arc::new(Schema::new(empty_fields));

[alamb]

I think we should also

1. check that the schema is correctly read back
2. Verify that the reader works correctly (reading back no batches)

For example, something like:

Suggested change

	// read from parquet
	let bytes = Bytes::from(parquet_bytes);
	let result = ParquetRecordBatchReaderBuilder::try_new(bytes);
	result.unwrap();
	// read from parquet
	let bytes = Bytes::from(parquet_bytes);
	let reader = ParquetRecordBatchReaderBuilder::try_new(bytes).unwrap();
	assert_eq!(reader.schema(), &empty_batch.schema());
	let batches: Vec<_> = reader.build().unwrap().collect::<ArrowResult<Vec<_>>>().unwrap();
	assert_eq!(batches.len(), 0);

(I ran this locally and it passed so I think we could also make these changes as a follow on PR)

[etseidl]

Thanks for the review @alamb. I made the changes you suggested.