Fix clean test drive throws when empty

When having StrictMode enabled and trying to clean up an empty TestDrive, the Context function throws a PropertyNotFoundException.
````
Set-StrictMode -Version 2
Clear-TestDrive -Exclude ($TestDriveContent).FullName
````
The TestDrive is empty so $TestDriveContent is $Null
````
Clear-TestDrive -Exclude ($Null).FullName
````
which causes the exception.

I have added a simple sanity check that should prevent that problem.
PS: Sorry, I could not figure out a way to write a Unit Test for it (as Context would then be using itself)