Prevent alter_column() from changing nullability

Prevent alter_column() from changing nullability

Fixed bug where an alter_column() operation would change a "NOT NULL"
column to "NULL" by emitting an ALTER COLUMN statement that did not specify
"NOT NULL". (In the absence of "NOT NULL" T-SQL was implicitly assuming
"NULL"). An alter_column() operation that specifies `type_=` should also
specify either `nullable=` or `existing_nullable=` to inform Alembic as to
whether the emitted DDL should include "NULL" or "NOT NULL"; a warning is
now emitted if this is missing under this scenario.

Fixes: #977
Change-Id: Ifbaa06bf149ba39d1a5deb43a6fd42c9ca118894