Fix bug in purple-list checks for Pythia

Description of bug

In the new check for the purple-listing implemented in !1759 (merged) the code would crash if pl_reason was none.

Changes introduced

Before checking if CKKW is in pl_reason check if pl_reason actually exists (i.e. the release is actually purple-listed)

Tests

Before fix

After fix

Issues resolved

Issue in !1811 (merged)