Fix handling of None as parameter for a datetimeoffset column

Fix handling of None as parameter for a datetimeoffset column

Fixed issue where the :class:`.mssql.DATETIMEOFFSET` type would not
accommodate for the ``None`` value, introduced as part of the series of
fixes for this type first introduced in :ticket:`4983`, :ticket:`5045`.
Additionally, added support for passing a backend-specific date formatted
string through this type, as is typically allowed for date/time types on
most other DBAPIs.

Fixes: #5132
Change-Id: Iab05d67382e0f550474d50e0c3c1c888521b678a