Return Nones from *_from_iso tool functions

The today/now defaults were leading to buggy and unexpected data so
return None instead.

metrics/tools/dates.py

@@ -1,19 +1,19 @@
-from datetime import date, datetime, timedelta
+from datetime import datetime, timedelta
 
 
 DELTA = timedelta(days=1)
 
 
 def date_from_iso(value):
     if value is None:
-        return date.today()
+        return None
 
     return datetime_from_iso(value).date()
 
 
 def datetime_from_iso(value):
     if value is None:
-        return datetime.now()
+        return None
 
     return datetime.fromisoformat(value)