replace 0000-00-00 dates by NULL

[zormit]

This is a proposition to solve issue #15.

It has to be discussed, if it makes more sense to replace "zero"-date by NULL or by some random value like somewhere around the start of the epoch. When there is no NOT NULL constraint, I think it's better to replace by NULL, as every other date could interfere with the regularly stored dates.

[mihailShumilov]

Null is not good way
At some case timestamp field must be required
And null value throw error

[zormit]

As describe in #15, postgres won't accept 0000-00-00, so something should be done, even if not using my proposed solution.

An option would be to fill it e.g. with 1900-01-01, which I find a bit random but would prevent at least the case of having a NULL value.