Update Start-RSJob.ps1

[JacobStruiksma]

Check if argument is not null before checking its type

Fixes #189 .

Changes proposed in this pull request:

• Check if argument is not null before showing its type

How to test this code:

Start-RSJob -ArgumentList 'x',$null -ScriptBlock {
    param(
        [string]$FirstInput,
        $DataInput
    )   
}

This should not show an error

Has been tested on (remove any that don't apply):

• Powershell 5
• Windows 10