[sqlalchemy-20] Use a ``TextClause`` object for the empty strings in the DB model definitions

[Rodolfo Alonso]

The alembic comparator between the DB models and the loaded metadata in the database, cannot compare an empty string clause with a ``TextClause``. For example:
[( 'modify_default',
      None,
      'ml2_port_bindings',
      'vif_details',
      { 'existing_comment': None,
        'existing_nullable': False,
        'existing_type': VARCHAR(length=4095)},
      DefaultClause(<sqlalchemy.sql.elements.TextClause object at 0x7fbadc92ada0>, for_update=False),
      DefaultClause('', for_update=False))],

This is happening because in the DB model definitions we are using an empty string. For example, from the error presented, the "vif_details" column definition in the "ml2_port_bindings" table is [1]. The "server_default" parameter should be a ``TextClause`` object instead.

[1]https://github.com/openstack/neutron/blob/a26e957d346c79ce96680b437460c9a9c4b80447/neutron/plugins/ml2/models.py#L50-L51

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/openstack/neutron/+/883421
Committed: https://opendev.org/openstack/neutron/commit/6fa3d8019ffd416ad3eb90e6e9a364d4b6e23e80
Submitter: "Zuul (22348)"
Branch: master

commit 6fa3d8019ffd416ad3eb90e6e9a364d4b6e23e80
Author: Rodolfo Alonso Hernandez <email address hidden>
Date: Wed May 17 19:26:50 2023 +0200

    Use ``TextClause`` to define the DB model "server_default"

    The alembic comparator needs the same class to compare the
    database model with the loaded SQL schema.

    Closes-Bug: #2020050
    Change-Id: I4feab3e55f2e38a9eca70d4ca755b4c2974346d5

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/neutron 23.0.0.0b3 development milestone.