Set stdio values to None when given an empty string

[mattisafur]

This will give an option output to the default device hen using inputs in launch.json.
For example:

{
    "version": "0.2.0",
    "configurations": [
        {
            "type": "lldb",
            "request": "launch",
            // ...
            "stdio": [
                null,
                "${input:stdoutFile}",
                null
            ],
            // ...
        }
    ],
    "inputs": [
        {
            "id": "stdoutFile",
            "type": "promptString",
            "description": "Path to STDOUT file",
        }
    ]
}

In this case, when stdoutFile is not set by the user stdio will be:

"stdio": [
    null,
    "",
    null
],

This will result in an error.

To counteract this, the change I made replaces empty strings with None.