Warnings fixes in DAL, XUnitTest and XIntegrationTest layers

[mythter]

• Ticket #1651

Code reviewers

• [ ] @sashapanasiuk5
• [x] @Mikhail-Kolpakov
• [ ] @ita-social-projects/734-08-streetcode

Summary of issue

• Check and resolve any warnings present in the application codebase (DAL, XUnitTest and XIntegrationTest layers). This includes identifying warnings reported by the compiler, static analysis tools, or any other code quality tools integrated into the development environment.

Summary of change

• Fixed warnings in the specified projects
• DAL MimeKit nuget package updated to 4.7.1
• Newss changed to News where needed
• Commented tests were uncommented and made functional (or partially)
• BLL all null checks in GetAll handlers changed to .Any() method check
• Fixed tests that started fail after BLL fixes

CHECK LIST

• [ ] СI passed
• [ ] Сode coverage >=95%
• [ ] PR is reviewed manually again (to make sure you have 100% ready code)
• [ ] All reviewers agreed to merge the PR
• [ ] I've checked new feature as logged in and logged out user if needed
• [ ] PR meets all conventions