Skip components without contours

Fix for the traceback below. It happens when `nbspace` glyph uses `space` glyph as component, for example.

```python
Traceback (most recent call last):
  File "lib/eventTools/eventManager.pyc", line 116, in callObserver_withMethod_forEvent_withInfo
  File "glyphNanny.py", line 88, in drawReport
  File "lib/fontObjects/robofabWrapper.pyc", line 3188, in getRepresentation
  File "/Applications/RoboFont.app/Contents/Resources/lib/python2.7/defcon/objects/glyph.py", line 641, in getRepresentation
  File "glyphNanny.py", line 423, in GlyphNannyReportFactory
  File "glyphNanny.py", line 397, in getGlyphReport
  File "glyphNanny.py", line 674, in testComponentMetrics
TypeError: 'NoneType' object has no attribute '__getitem__'
```