Add error message for PowerShell adapter without input

[Gijsreyn]

PR Summary

The PowerShell adapter requires a check whenever input is empty, it should return a proper error message. In this case, the default @{} was given as a parameter without converting it to JSON. Now it checks the JSON input and if empty, converts it still to an empty JSON string.

[Gijsreyn]

@anmenaga Can you review this PR? It's a small one.

[anmenaga]

A generic fix for this issue is in #525 .