mySQL 5.7 nullDate compatibility fix

[krim404]

Summary of Changes

implemented the nullDate change for mysql 5.7 (from 0000-00-00 00:00:00 to 1000-01-01 00:00:00)

Testing Instructions

installation and/or updating on mysql 5.6 server, all datetime database default values of joomla should be 0000-00-00 00:00:00,

on installation and/or updating on mysql 5.7 server, all datetime database default values of joomla should be 1000-01-01 00:00:00

this could break external components, who rely on hardcoded nullDates.

[krim404]

this patch has been created at @icampus
it also implements #11527

the update routine should work in both ways (upgrade and downgrade of mysql version) - downgrade is experimental.

[krim404]

redo pull request, because of failed merge from github gui. Redoing from console.

New one #11530