* Before this change function task(task_name) available in Mistral
expressions would fail with an error if such task doesn't exist.
It makes impossible to have expressions like <% task(t1) != null %>
which is very convenient in cases when we need to gather meta
info about how a workflow was running and what tasks have already
started and what have not. So now it simply returns 'null'.
Change-Id: Ifd1ea9b5ee946c7072044d1402296710676845db
Closes-Bug: #1631035