WL-3170 Document the mail URI private method

Even though it is a private method it could do with some documentation to
stop people from panicking when we potentially give it `null`.
WL-3170 Organisers don't require emails

This makes sure that an Organiser will be defined even if there is no email
address defined for them.
WL-3170 Test new behaviour

Now that it should be possible to generate a calendar file where the
organiser doesn't have an email, let's define this in the test.
Merge pull request #7 from ox-it/WL-3170

WL-3170 Creator email can be blank