Allow values to to be missing

Map missing programming/open source/etc values to "(none)".
This can be then used in the right sections in grader.conf to assign
a value. I think limesurvey should be configured to not allow incomplete
values like that, but if it wasn't, then let's make the best of this situation.

[open_source_rating]
never used = 0.0
...
(none) = 0

[python_rating]
competent = 1.0
...
(none) = 0

[vcs_rating]
no = 0
...
(none) = 0

grader/grader.py

@@ -1203,7 +1203,10 @@ def get_rating(name, dict, key, fallback=None):
 
     Throws MissingRating if rating is not present.
     """
-    key = key.partition('(')[0].partition('/')[0].strip().partition(',')[0].strip()
+    if key == '':
+        key = '(none)'
+    else:
+        key = key.partition('(')[0].partition('/')[0].strip().partition(',')[0].strip()
     try:
         return dict[key]
     except KeyError:
@@ -1216,6 +1219,7 @@ def get_rating(name, dict, key, fallback=None):
     'male'      : 'M',
     'other'     : 'O',
     'non-binary': 'O',
+    ''          : 'U', # unknown
     'prefer not to say' : 'U'
 }
 def gender_to_formula_label(label):
@@ -1233,7 +1237,7 @@ def rank_person(person, formula, location,
         key = getattr(person, attr)
         value = get_rating(attr, dict, key)
         vars[attr] = value
-    vars.update(born=int(person.born), # if we decide to implement ageism
+    vars.update(born=int(person.born) if person.born else 0, # if we decide to implement ageism
                 gender=gender_to_formula_label(person.gender), # if we decide, …
                                                                # oh we already did
                 nonmale=person.nonmale,