Don't put an attribute into the database for 'url' that is left blank

Don't put an attribute into the database for 'url' that is left blank

The URL field, (for either a hunt or a puzzle), is an optional
parameter, (the user creating the hunt or puzzle may or may not
provide a URL). Immediately prior to this commit, if a user left the
URL field blank, the submission handler was receiving a value of None
and inserting that as an attribute in the database.

This was problematic as subsequent code, (such as sheet creation),
would be prepared for an item to have no url key at all, but was not
prepared for the key to be present but for the value to be empty.

We fix this in this commit by explicitly _not_ inserting a None value
for url as an attribute in the database item, (instead, not supplying
a url attribute at all).