Adjust mssql accommodate existing_type and type

Adjust mssql accommodate existing_type and type

Fixed bug where the "existing_type" parameter, which the MSSQL dialect
requires in order to change the nullability of a column in the absence of
also changing the column type, would cause an ALTER COLUMN operation to
incorrectly render a second ALTER statement without the nullability if a
new type were also present, as the MSSQL-specific contract did not
anticipate all three of "nullability", "type_" and "existing_type" being
sent at the same time.

Change-Id: Ia95b6c3e9276cc067fd773928f9678ab429d5670
Fixes: #812