Saas: Should define default connection string if shared database is selected. #18060
Description
Resolves #18027
@sumeyyeKurtulus I added the related validation for MVC & Blazor UIs, you can either add it in this PR for Angular or create a new PR. FYI.
Checklist
- I fully tested it as developer / designer and created unit / integration tests
- I documented it (or no need to document or I will create a separate documentation issue)
- I worked with the design team to get their idea for UI/UX design (or no need to design for this PR)
- I've assigned reviewer, related labels and set a milestone for this PR