Don't apply a length to reflected TEXT / NTEXT

Don't apply a length to reflected TEXT / NTEXT

Fixed issue in SQL Server reflection where ``TEXT`` and ``NTEXT``
columns would be reflected with a spurious length of 16 and 8,
respectively.  These are unlengthed LOB datatypes; the value
originates from the ``sys.columns.max_length`` column, which reports
the size of the in-row LOB pointer rather than a character length for
these types.  The reflected ``TEXT`` and ``NTEXT`` types now have a
``length`` of ``None``, so that a reflected table emits valid DDL when
re-created, which previously failed with "Cannot specify a column
width on data type text".

Fixes: #13451
Change-Id: I8456688fc8d21fe25f326c6bf0f7b13aa1fc838c
(cherry picked from commit 325f71701a8a994c982c781f59bf34f42add88e7)