time: Add IsNull() function for Time struct

[b-s-a]

Current Time structure has no method like IsNull() (Valid() or something else) so you need to use t.Equal(time.Time{}) or pointers to check is time defined or not (for example, when you aquire values from external sources and some of them may be omited). Usage of pointers for such small object has known overhead, so I propose to add function IsNull() which returns true for uninitialized time structure (1970-01-01 00:00:00).

[agnivade]

https://golang.org/pkg/time/#Time

The zero value of type Time is January 1, year 1, 00:00:00.000000000 UTC. As this time is unlikely to come up in practice, the IsZero method gives a simple way of detecting a time that has not been initialized explicitly.

[b-s-a]

Oh sorry. I many times tries to find IsNull, but cannot find IsZero. Thank you.