eds: Fix crash on handling invalid birthday dates from contacts

If a GDateTime cannot be constructed due to the requested date being
invalid or out of range, the constructor will return null. The code was
not previously handling this.

https://bugzilla.gnome.org/show_bug.cgi?id=727944
telepathy: Fix crash on handling invalid birthday dates from contacts

If a GDateTime cannot be constructed due to the requested date being
invalid or out of range, the constructor will return null. The code was
not previously handling this.

https://bugzilla.gnome.org/show_bug.cgi?id=727944
tracker: Fix crash on handling invalid birthday dates from contacts

If a GDateTime cannot be constructed due to the requested date being
invalid or out of range, the constructor will return null. The code was
not previously handling this.

https://bugzilla.gnome.org/show_bug.cgi?id=727944
tests: Add assertions to check for null DateTimes

If the DateTime would end up being invalid, construction fails and null
is returned. Gracefully catch that in the unit tests.

https://bugzilla.gnome.org/show_bug.cgi?id=727944