Don't log error if no files unloaded

[maxim-lisovsky-gismart]

I'm using parquet files for unload_and_copy, unload options: ["PARQUET", "PARALLEL ON", "CLEANPATH"]
I get S3DownloadError and an error log in case my query returns no rows:

ERROR: No files generated from unload

I believe that it's no need for throwing an exception and logging an error when using parquets

[fdosani]

@maxim-lisovsky-gismart Thanks for flagging this. I'll take a closer look shortly.