.Net: Further improvement to MongoDB reliability. (#11857)

### Motivation and Context

When the mongodb container initialization fails, the disposal also
failed, so fixing this issue.

### Description

<!-- Describe your changes, the overall approach, the underlying design.
These notes will help understanding how your code works. Thanks! -->

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [ ] The code builds clean without any errors or warnings
- [ ] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [ ] All unit tests pass, and I have added new tests where possible
- [ ] I didn't break anyone 😄

Author: westey-m

dotnet/src/IntegrationTests/Connectors/Memory/MongoDB/MongoDBVectorStoreFixture.cs

@@ -4,6 +4,7 @@
 using System.Collections.Generic;
 using System.Threading;
 using System.Threading.Tasks;
+using DotNet.Testcontainers.Containers;
 using Microsoft.Extensions.VectorData;
 using MongoDB.Driver;
 using Testcontainers.MongoDb;
@@ -68,16 +69,22 @@ public async Task InitializeAsync()
 
     public async Task DisposeAsync()
     {
-        var cursor = await this.MongoDatabase.ListCollectionNamesAsync();
-
-        while (await cursor.MoveNextAsync().ConfigureAwait(false))
+        if (this.MongoDatabase is not null)
         {
-            foreach (var collection in cursor.Current)
+            var cursor = await this.MongoDatabase.ListCollectionNamesAsync();
+
+            while (await cursor.MoveNextAsync().ConfigureAwait(false))
             {
-                await this.MongoDatabase.DropCollectionAsync(collection);
+                foreach (var collection in cursor.Current)
+                {
+                    await this.MongoDatabase.DropCollectionAsync(collection);
+                }
             }
         }
 
-        await this._container.StopAsync();
+        if (this._container is not null && this._container.State == TestcontainersStates.Running)
+        {
+            await this._container.StopAsync();
+        }
     }
 }