ArgumentNullException on short deployment-env

## Description

I got some errors because of names too long for resources and tried to shorten them by using "D" as deployment-env. As a result I got an ArgumentNullException (see stacktrace below).

## Environment and tool versions

- AIM Tool Version (aim --version): 0.5.1-beta.2020102844785
- Windows Version (systeminfo): Win10
- Shell: Powershell
- Shell Version ($PSVersionTable | bash --version):

PSVersion 7.1.3
PSEdition Core
GitCommitId 7.1.3
OS Microsoft Windows 10.0.18363
Platform Win32NT
PSCompatibleVersions {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion 2.3
SerializationVersion 1.1.0.1
WSManStackVersion 3.0

## Repro steps

run the tool using options like:

aim migrate -a "microsoft.biztalk.msidiscoverer.msidir=..\msi" --deployment-env d -v

## Expected result

Either success or a more descriptive error message stating any restrictions for the deployment-env parameter.

## Current result

ArgumentNullException