datetime: allow GstDateTime where not all fields are set

datetime: allow GstDateTime where not all fields are set

In order to deserialise and re-serialise dates and date times
from tags properly, we need to be able to express partial
dates (e.g. YYYY or YYYY-MM) and date times.

We only support partial date times where all the more
significant fields above the first unset field are set
(e.g. YYYY-00-DD is not supported).

Calling _get_foo() when foo is not set is not allowed
any more, callers need to check which fields are set
first.

https://bugzilla.gnome.org/show_bug.cgi?id=677757