Don't fail on events with no dtend

[madjar]

I ran the parser on my google calendar export (after the fixes from #2), and I noticed that I have events with a DTSTART and no DTEND, which the parser doesn't like. This PR ensures that we can resolve the nil dtend to nil :)

I couldn't find place where a test would fit, but I'm happy to add one if you point me in the right direction.

[jonathanballs]

Hi @madjar,

Thanks for the PR - that's a good catch. I suggest throwing something like this at the bottom of test/magical/parser_test.exs. Then I'd be happy to merge :)

  describe "time zone resolution" do
    test "gracefully handles nil start or end times" do
      {:ok,
       %Magical.Calendar{
         events: [
           %Magical.Event{
             dtstart: ~D[2018-01-01],
             dtend: nil
           }
         ]
       }} =
        """
        BEGIN:VCALENDAR
        BEGIN:VEVENT
        DTSTART;VALUE=DATE:20180101
        END:VEVENT
        END:VCALENDAR
        """
        |> Parser.parse()
    end
  end

[jonathanballs]

I noticed that you have found a couple of extra edge cases that you mentionned in the other PR. I will review them shortly.

[madjar]

Updated with the test!

I tweak the one you provided a bit, since the test ics needs to have a timezone to enable that code path. I also went with a datetime to exercise the conversion behaviour.

[jonathanballs]

Merged - thank you for your contribution! I will wait until the other PR for alarms is merged before pushing a new hex release 👍

[madjar]

Thanks!