Fixed crash when cloud shell provider timed out or was closed waiting…

… for login (#16364)

## Summary of the Pull Request
Cloud shell connection calls out to Azure to do a device code login.
When the polling interval is exceeded or the tab is closed, the method
doing the connection polling returns `nullptr`, and `AzureConnection`
immediately tries to `GetNamedString` from it, causing a crash. This
doesn't repro on Terminal Stable or Preview, suggesting it's pretty
recent related to the update of this azureconnection.

This is just a proposed fix, not sure if you want to do more extensive
changes to the affected class or not, so marking this as a draft.

## References and Relevant Issues
* N/A - encountered this while using the terminal myself

## PR Checklist/Validation
Tested out a local dev build:

- [x] Terminal doesn't crash when cloudshell polling interval exceeded
- [x] Terminal doesn't crash when cloudshell tab closed while polling
for Azure login

(cherry picked from commit 0c4751b)
Service-Card-Id: 91232783
Service-Version: 1.18

src/cascadia/TerminalConnection/AzureConnection.cpp

@@ -604,6 +604,15 @@ namespace winrt::Microsoft::Terminal::TerminalConnection::implementation
 
         // Wait for user authentication and obtain the access/refresh tokens
         auto authenticatedResponse = _WaitForUser(devCode, pollInterval, expiresIn);
+
+        // If user closed tab, `_WaitForUser` returns nullptr
+        // This also occurs if the connection times out, when polling time exceeds the expiry time
+        if (!authenticatedResponse)
+        {
+            _transitionToState(ConnectionState::Failed);
+            return;
+        }
+
         _setAccessToken(authenticatedResponse.GetNamedString(L"access_token"));
         _refreshToken = authenticatedResponse.GetNamedString(L"refresh_token");