Remove trailing empty argument in ContainerCommand

The New-AzContainerGroup -Command argument adds an extra empty argument to the end of the commands. This is because Parser.ParseInput will return <EOF> as the final token. This means any programs using command arguments will get an extra empty argument passed in. The Python-based az cli does not do this. To repro: 

1. Create an app that shows how many arguments were passed into it. For example in .net core:  
```
class Program
{
        static void Main(string[] args)
        {
            Console.WriteLine("I have {0} arguments", args.Length);
        }
}
```
2. Add this to a container and push to a registry. 
3. Run the container as an ACI using powershell
```
New-AzContainerGroup -ResourceGroupName test -Name bugtest -Image showargImage -OsType Linux -Command "dotnet showargs.dll one two three"
```
4. Examine the logs:
```
Get-AzContainerInstanceLog -ResourceGroupName test -ContainerGroupName bugtest
I have 4 arguments
```
5. Run the image using the az CLI
```
az container create --resource-group test --name bugtest2 --image showargImage --command-line "dotnet BugTest.dll one two three"
``` 
6. Examine the output
```
az container logs --resource-group akstest --name bugtest2
I have 3 arguments
```

Author: ssemyan

src/ContainerInstance/ContainerInstance/Commands/NewAzureContainerGroupCommand.cs

@@ -263,7 +263,7 @@ public override void ExecuteCmdlet()
                     {
                         throw new ArgumentException($"Invalid 'Command' parameter: {string.Join("; ", errors.Select(err => err.Message))}");
                     }
-                    creationParameter.ContainerCommand = tokens.Select(token => token.Text.Trim('\'', '"')).ToList();
+                    creationParameter.ContainerCommand = tokens.Select(token => token.Text.Trim('\'', '"')).Where(token => !string.IsNullOrEmpty(token)).ToList();
                 }
 
                 creationParameter.Validate();