When a TestCase object has no action_metadata associated with it, the
object attribute is None.  Check if this is the case before calling
dict methods on it, to fix exceptions like this one:

  [...]
    File "/usr/lib/python3/dist-packages/lava_results_app/dbutils.py", line 352, in walk_actions
      build_action(action, testdata, submission)
    File "/usr/lib/python3/dist-packages/lava_results_app/dbutils.py", line 332, in build_action
      if 'level' in case.action_metadata:
  TypeError: argument of type 'NoneType' is not iterable

Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>