Unwrap TIMESTAMP when doing an isinstance()

Unwrap TIMESTAMP when doing an isinstance()

Fixed bug where the special logic to render "NULL" for the
:class:`.TIMESTAMP` datatype when ``nullable=True`` would not work if the
column's datatype were a :class:`.TypeDecorator` or a :class:`.Variant`.
The logic now ensures that it unwraps down to the original
:class:`.TIMESTAMP` so that this special case NULL keyword is correctly
rendered when requested.

Fixes: #4743
Change-Id: I02b22dfa3db06daea37b044e2206a8569e2e5d22
(cherry picked from commit 401a2691fbeebd5a26341e732644584cb096bc58)