Ensure jobparams and group_jobparams are dict

When defining a list of jobs in a project, if a colon is added after
the job name but no parameters is provided, jobparams is None.

As dict.update() expects parameter to be iterable, ensure jobparams
and group_jobparams are always a dict to prevent JJB from crashing.

Change-Id: Ie57aa9e5b446fe502dff896cde773ddcc92ba803
Closes-bug: #1253180
Merge "Ensure jobparams and group_jobparams are dict"