Fix null reference exception when parameters not passed to script.

A null reference exception would be thrown if a PowerShell script file
containing a param script block, with one or more parameters, was run
and no parameters were passed in the command line.

   PASH> MyScript.ps1

MyScript.ps1 file:

    param ($a)
    ...
Merge pull request #315 from mrward/fix-undefined-script-params-null-ref

Fix null reference exception when parameters not passed to script.