[Minor], Bug Fix: Add empty ordering check at the source.

[mustafasrepo]

Which issue does this PR close?

Closes #.

Rationale for this change

At the projection of the source, we keep track of ordering as long as they are used in the query.
Consider table below

CREATE EXTERNAL TABLE multiple_ordered_table (
  a0 INTEGER,
  a INTEGER,
  b INTEGER,
  c INTEGER,
  d INTEGER
)
STORED AS CSV
WITH HEADER ROW
WITH ORDER (a ASC, b ASC)
WITH ORDER (c ASC)
LOCATION 'tests/data/window_2.csv';

which satisfies ordering a ASC, b ASC and c ASC.
If we run the query below on this table.

SELECT c
FROM multiple_ordered_table
ORDER BY c ASC;

since, only column c is used. We will not keep track of ordering a ASC, b ASC. However, during projection of orderings
empty vectors are still added to output ordering. Since, first entry in among alternative orderings is output_ordering. Output ordering of the CsvExec is empty.
In this PR, we add emptiness check, with this check we no longer store empty orderings(after projection) at the source. After this PR output ordering of the CsvExec is no longer empty.

What changes are included in this PR?

Are these changes tested?

Yes, new tests are added.

Are there any user-facing changes?

[alamb]

Thanks @mustafasrepo

[alamb]

👍