fix: in alembic's offline mode (db migrate with --sql option), skip data operations

[sagarey]

Description

In some cases where database DDL is limited, SQL operations are controlled independently. Migrations cannot run in the deployment environment, require Alembic generate raw SQL

flask db upgrade --sql triggered the error.

Type of Change

• [X] Bug fix (non-breaking change which fixes an issue)
• [X] This change requires a documentation update, included: Dify Document Maybe the docs could mention that when deployment migrations don't work, can use Alembic offline mode to get the raw SQL.

How Has This Been Tested?

• [X] Testing what the --sql parameter does.

• [X] Testing the migration that's causing the error.

• [X] Testing the complete migration.

Suggested Checklist:

• [X] I have performed a self-review of my own code

• [X] I have commented my code, particularly in hard-to-understand areas

• [X] My changes generate no new warnings

• [X] I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

• [ ] optional I have made corresponding changes to the documentation

• [ ] optional I have added tests that prove my fix is effective or that my feature works

• [ ] optional New and existing unit tests pass locally with my changes

[bowenliang123]

Nice feature. Reasonable and practical for enterprise deployment and migration.