PR Summary

This pull request adds a simple input validation when it's empty for the GetOperation on the Repository resource.

PR Context

When --input is omitted, DSC closes the child process stdin without writing anything. The
resource manifest uses "input": "stdin" and invokes the script as $Input | ./psresourceget.ps1 ..., so $Input is an empty enumerator and the $stdinput pipeline parameter is never bound (keeping it $null).

Fixes #2001

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