Order of the series sales in PostgreSQL shows records without date first

[php-coder]

Addition of new tests in 52df7f3 (#498), reveal a difference in the order of sales in the series list -- it shows sales without dates (NULL) first on PostgreSQL.

Here is how the test fails:

FAIL: Robotframework.Series.Sales.Creation.Logic.Add a sale with all fields
The text of element 'id:series-sale-1-info' should have been '04.01.2021 Eicca Toppinen sold to Tommy Lee Jones for 7.50 EUR (10.10 USD) (MNH)', but it was 'Tommy Lee Jones was selling for 125.00 RUB'.

Seen there: https://travis-ci.com/github/php-coder/mystamps/jobs/468143974

I expect that the order it the same despite what the database is being used.

[php-coder]

Related links:

• https://stackoverflow.com/questions/9510509/sort-by-column-asc-but-null-values-first
• https://stackoverflow.com/questions/2051602/mysql-orderby-a-number-nulls-last