NullDates are different across databases

### Issue
Since the various databases have different null dates the check in the JHtml::calendar failes on non-MySQL databases. The result is that if you edit for example an article and save it again, the "Finish Publishing" date will be populated with "1970-01-01" and the article is expired and no longer shows.
See https://groups.google.com/d/msg/joomla-dev-cms/R8iw4TDgbZo/MmTUcIfoOMEJ for the original issue message.

### Testing
Test if the date fields act as expected both when there is a value saved and if left empty.