Experimental `PSRedirectToVariable` feature: target variable isn't cleared if the captured stream has no output

Prerequisites

• Write a descriptive title.
• Make sure you are able to repro it on the latest released version
• Search the existing issues.
• Refer to the FAQ.
• Refer to Differences between Windows PowerShell 5.1 and PowerShell.

Steps to reproduce

The experimental PSRedirectToVariable feature conveniently allows capturing stream output via > and >> in variables, e.g. >variable:stdout.

If the targeted stream has no output, the variable should be cleared to indicate that fact (if appending is desired, >> can be used).
However, any preexisting variable value currently unexpectedly lingers.

# Note: Enable-ExperimentalFeature PSRedirectToVariable must be in effect
# (turning it on requires starting a new session.)
$stdout = 'psych!'
whoami -nosuch >variable:stdout 2>$ull
$stdout

Expected behavior

No output, because no stdout output was received, which should set $stdout to the enumerable null ([System.Management.Automation.Internal.AutomationNull]::Value), whether or not the variable previously existed.

This would be consistent with the behavior of direct assignments; e.g., $stdout = & {} properly records the enumerable null.

Actual behavior

psych!

That is, the preexisting value of the target variable unexpectedly lingered.

Error details

No response

Environment data

PowerShell 7.5.0-rc.1

Visuals

No response

Enable the PSRedirectToVariable experimental feature and run the reproduction using $stdout, whoami -nosuch, and >variable:stdout. Trace the PSRedirectToVariable handling for a stream with no output, then verify that an existing target variable becomes the enumerable null and that the append behavior remains distinct.