Figure out how to understand if $null is passed to callback

https://github.com/KnicKnic/native-powershell/blob/c2c84066c3b1f0da5b49dfc1c405be8fa37a2ac9/host.cpp#L248-L258

# repo steps
## run unit tests
see https://github.com/KnicKnic/native-powershell#running-the-unit-tests
## set breakpoints
in above code line 257
and below code in lines 218 and 223
https://github.com/KnicKnic/native-powershell/blob/c2c84066c3b1f0da5b49dfc1c405be8fa37a2ac9/host.cpp#L211-L227

## see test case
"nullptr | send-hostcommand"
https://github.com/KnicKnic/native-powershell/blob/c2c84066c3b1f0da5b49dfc1c405be8fa37a2ac9/test_unit/test_unit.cpp#L189-L200

Run the unit tests and reproduce "nullptr | send-hostcommand" from test_unit/test_unit.cpp lines 189-200. Set breakpoints at host.cpp lines 257, 218, and 223, then trace how the callback receives the null value. Done means establishing how to reliably detect that $null was passed and documenting the required behavior.

Written by the indexing model from the issue text.