(#3765) Don't try to decrypt cert password

[corbob]

Description Of Changes

When attempting to decrypt a string that is null or empty, instead of attempting to decrypt, return null.

Motivation and Context

It is not possible to decrypt a string that doesn't exist. This also brings the decrypt logic in line with the encrypt logic that already included this check.

Testing

1. Setup a Repository that accepts a certificate (https://docs.chocolatey.org/en-us/guides/organizations/set-up-certificate-authentication/#using-sonatype-nexus for using Nexus through nginx).
2. Add the new source with a passwordless certificate
3. Attempt to use the source

Internal Chocolatey Software testing:

1. Follow the testing found here. Add to the testing using userNoPassword.pfx as well as userWithPassword.pfx in your testing.

Operating Systems Testing

Windows 11

Change Types Made

• Bug fix (non-breaking change).
• Feature / Enhancement (non-breaking change).
• Breaking change (fix or feature that could cause existing functionality to change).
• Documentation changes.
• PowerShell code changes.

Change Checklist

• Requires a change to the documentation.
• Documentation has been updated.
• Tests to cover my changes, have been added.
• All new and existing tests passed?
• PowerShell code changes: PowerShell v3 compatibility checked?

Related Issue

• Fixes Message about value cannot be null appears when using a locally defined source #3765

[corbob]

I've opened this PR as a draft as I would really like to get some tests around certificate authentication, but need to think a bit on this.

[corbob]

I'm not sure yet, but I think this might be related to #2736 (as in, this fix fixes that issue too). Will need to do more investigating.

[corbob]

@gep13 I've marked this PR ready for review, and requested the review from you due to the linked testing PR also being assigned to you.