[FEATURE] Enable TimeSpan work with only start Time

[calien666]

First of all, thank you for your great package.

Implementing your package at a customer leads to the issue, that handling with Events having only a start time, but no end time set, is not possible.

I first used next midnight to solve this, but customer only wants the start time and date set for these events.

I checked a corresponding event in the iCal validator and having an event only set with a DTSTART is a valid behaviour.

In your package the only class adopting this case, is TimeSpan, which needs to be begin and end set as DateTime.

Expected behaviour

Allow TimeSpan creation with only start set.