widget: Don't segfault with JSON files containing unknown properties
widget: Don't segfault with JSON files containing unknown properties

When the JSON file has a property that does not have a param spec, the
default implementation in MxWidget checking for the translatable flag
segfaulted.

clutter-project/mx,clutter-project/mx,pwithnall/mx,clutter-project/mx,pwithnall/mx,pwithnall/mx,jonnylamb/mx,jonnylamb/mx,jonnylamb/mx,pwithnall/mx,clutter-project/mx,jonnylamb/mx