Fix columns with falsy cells wrongly parsed as dates (#276)

Eg. before this commit, this table would result in Date columns:

| A     | B |
| ----- | -- |
| FALSE | 0 |

For now, even FALSE is parsed as Numeric (not sure why we don't have
a BooleanConverter).

sandbox/grist/parse_data.py

@@ -83,7 +83,7 @@ class SimpleDateTimeConverter(BaseConverter):
   def convert(cls, value):
     if type(value) is datetime.datetime:
       return value
-    elif not value:
+    elif value is None:
       return None
     raise ValueError()