Fix segfault issue from empty metadata

[alinaliBQ]

Transform_inner was having a seg fault error because metadata() return result was not being checked for null value. I see in flight_sql/flight_sql_result_set_metadata.cc there is already a check for null metadata(), so I re-used this check in Transform_inner.

arrow/cpp/src/arrow/flight/sql/odbc/flight_sql/flight_sql_result_set_metadata.cc

Lines 45 to 53 in a6bc579

	std::shared_ptr<const arrow::KeyValueMetadata> empty_metadata_map(
	new arrow::KeyValueMetadata);
	inline arrow::flight::sql::ColumnMetadata GetMetadata(
	const std::shared_ptr<Field>& field) {
	const auto& metadata_map = field->metadata();
	arrow::flight::sql::ColumnMetadata metadata(metadata_map ? metadata_map
	: empty_metadata_map);

• Also fixed: Use metadata.GetTypeName() from flightsql-odbc

[jduo]

Let's change the ColumnMetadata object itself to handle NULL safely and internally create an empty owned map (since it's easy for this to occur elsewhere)

[alinaliBQ]

yea that makes sense, I have updated the code to change ColumnMetadata.

There is an API Field::HasMetadata in arrow that checks if metadata is null, but seems it's not as commonly used.

arrow/cpp/src/arrow/type.cc

Lines 304 to 306 in a6bc579

	bool Field::HasMetadata() const {
	return (metadata_ != nullptr) && (metadata_->size() > 0);
	}