Policy documents results table - sort by year newest > oldest shows 'year unknown' results before newest

[dd207]

As a user, when I sort results from newest to oldest in policy document results table, I want to see policy documents sorted from 2020 > older.

Currently I see 'unknown year' policy documents first, and then documents from 2020 on later pages. See screenshots below for example.

Showing documents from 'unknown year' first may confuse user and erode trust in the product.

This issue isn't a dealbreaker, we should go ahead and release this week. But I think it's important for there to be fix for this pushed soon after.

@SamDepardieu could you estimate how long it would take to fix?

Tagging @aoifespenge for info.