When no mail providers are registered, fall back to no-op behavior

[Tyrrrz]

705 changed the behavior in regard to how the mail providers are configured:

• Pre-PR, when the application configuration does not specify any mail provider, the emails are not sent.
• Post-PR, when the application configuration does not specify any mail provider, the FileMailProvider is used.

The issue is that, in certain scenarios, the application may not be able to write data to the file system and thus may not use the FileMailProvider, making it a poor default choice. This happens, for example, when running in Docker, where the application is executed under an under-privileged user account for security reasons:

Additionally, there does not appear to be a way to set the NoopMailProvider through configuration as it's only defined in tests.

This PR reverts some of the user-facing behavior changes introduced in #705 and makes it so that no emails are sent when the configuration does not specify any mail provider.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 80.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 35.12%. Comparing base (5bcf203) to head (b106d59). Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Common/Services/Mail/MailBootstrap.cs	33.33%	2 Missing :warning:
src/Common/Services/Mail/AggregateMailProvider.cs	90.90%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #714 +/- ## ========================================== - Coverage 35.14% 35.12% -0.03% ========================================== Files 574 574 Lines 31176 31155 -21 Branches 930 929 -1 ========================================== - Hits 10958 10942 -16 + Misses 20072 20067 -5 Partials 146 146 ```

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