Fixed PostgresToGCSOperator fail on empty resultset for use_server_side_cursor=True

[kazanzhy]

Attribute description of simple psycopg2 cursor is available after .execute() method.
But for the named cursor (server-side cursor) any .fetch* method call is needed.

In case of an empty result set like .execute("SELECT 1 LIMIT 0"):

• for the simple cursor, there are initialized .description and an empty iterator
• for the server-side cursor, there are not initialized .descriptionand an empty original iterator.
  But also we have self.rows=[None], as a result of fetchone() on an empty iterator of the server-side cursor.
  This None makes _PostgresServerSideCursorDecorator non-empty iterator.

I decided to remove rows attribute because it could consist of a maximum of one row, and also psycopg2 server-side cursor have .scroll() method and it is safe to call .scroll() on empty resultset even without .fetchone()

closes #20007

[kazanzhy]

If .scroll(-1) is more clear then I change

[uranusjr]

I like your current approach better.

[kazanzhy]

Sorry for the misleading.

First of all server-side cursor doesn't support relative mode by default. It should be initialized like conn.cursor(name='...', scrollable=True). But absolute mode worked without initialization.

But both scrolls cause a reset of .description attribute. So, unfortunately, they could not be used.

[uranusjr]

Actually, I think _PostgresServerSideCursorDecorator could be removed if

Are these existing tests to verify the behaviour of use_server_side_cursor? This suggested change makes sense to me but maybe I missed some cases expected by users.

[kazanzhy]

Today I made few tests:

Current version (code from main branch):

• Query returns rows and use_server_side_cursor=False: file with data
• Query returns rows and use_server_side_cursor=True: file with data
• Query returns nothing and use_server_side_cursor=False: empty file
• Query returns nothing and use_server_side_cursor=True: ERROR

Updated version (this PR code):

• Query returns rows and use_server_side_cursor=False: file with data
• Query returns rows and use_server_side_cursor=True: file with data
• Query returns nothing and use_server_side_cursor=False: empty file
• Query returns nothing and use_server_side_cursor=True: empty file

The last one is not what is expected in the issue but I think creating the empty file is the right behavior

I'm expected, that operator on empty table not creating file and no upload it on Google Cloud.