Manually initialize the warnings vector to prevent a crash on launch (#…

…8995)

## Summary of the Pull Request

Oops, winrt `IVector`s need to be manually initialized, when default-constructed `std::vector`s didn't. Simple oversight.

## PR Checklist
* [x] Closes #8986
* [x] I work here
* [x] A test would be great but ain't nobody got time for that.
* [n/a] Requires documentation to be updated

## Validation Steps Performed
Ran the terminal with 
```json

    "schemes" :
    [ {} ]
```
First the crash repro'd, now it doesn't.

src/cascadia/TerminalApp/TerminalPage.cpp

@@ -2410,7 +2410,7 @@ namespace winrt::TerminalApp::implementation
                                                                 IVectorView<Profile> profiles,
                                                                 IMapView<winrt::hstring, ColorScheme> schemes)
     {
-        IVector<SettingsLoadWarnings> warnings;
+        IVector<SettingsLoadWarnings> warnings{ winrt::single_threaded_vector<SettingsLoadWarnings>() };
 
         std::vector<ColorScheme> sortedSchemes;
         sortedSchemes.reserve(schemes.Size());