Fix warning with empty search results

Traceback (most recent call last):
File "/home/hadess/Projects/gnome-install/lib64/python2.7/site-packages/gnome_abrt/views.py", line 69, in problems_filter
  return match_pattern(pattern, model[itrtr][2])
File "/home/hadess/Projects/gnome-install/lib64/python2.7/site-packages/gnome_abrt/views.py", line 55, in match_pattern
  if item_match(pattern, problem) or pattern in problem.problem_id:
File "/home/hadess/Projects/gnome-install/lib64/python2.7/site-packages/gnome_abrt/views.py", line 51, in item_match
  val = problem[i].encode('utf-8')
AttributeError: 'NoneType' object has no attribute 'encode'