support building varfont from designspaces without <instances>

If one makes a minimal designspace (eg. with SuperPolator) for the
purpose of generating a variable font from a set of UFO masters,
one may have no instances defined. We need to account for the
instances element not be present in the designspace, instead of
failing with 'KeyError'.

Lib/fontmake/font_project.py

@@ -547,7 +547,7 @@ def _designspace_locations(self, designspace_path):
 
  maps = []
  ds = varLib.designspace.load(designspace_path)
- for location_list in (ds['sources'], ds['instances']):
+ for location_list in (ds['sources'], ds.get('instances', [])):
  location_map = {}
  for loc in location_list:
  abspath = os.path.normpath(os.path.join(