avoid error on empty workflow params

[ulferts]

Ticket

https://community.openproject.org/wp/63514

What are you trying to accomplish?

Be robust when sending an empty work flow form.

In such a case, the params["status"] is not present so the call to to_unsafe_h fails.

[ulferts]

The failing feature spec

rspec ./spec/features/notifications/notification_center/split_screen_spec.rb:55

cannot have been caused by the change within this PR.