Add empty config value for scope based config setting

[sureshanaparti]

Description

This PR add empty config value for scope based config setting.

Fixes: #5545

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)
• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Enhancement (improves an existing feature and functionality)
• Cleanup (Code refactoring and cleanup, that may add test cases)

Feature/Enhancement Scale or Bug Severity

Feature/Enhancement Scale

• Major
• Minor

Bug Severity

• BLOCKER
• Critical
• Major
• Minor
• Trivial

Screenshots (if appropriate):

How Has This Been Tested?

Manually tested empty config values in 'Domain' scope.

[sureshanaparti]

@blueorangutan package

[blueorangutan]

@sureshanaparti a Jenkins job has been kicked to build packages. I'll keep you posted as I make progress.

[blueorangutan]

Packaging result: ✔️ el7 ✔️ el8 ✔️ debian ✔️ suse15. SL-JID 1512

[sureshanaparti]

@blueorangutan test

[blueorangutan]

@sureshanaparti a Trillian-Jenkins test job (centos7 mgmt + kvm-centos7) has been kicked to run smoke tests

[nvazquez]

Code LGTM

[blueorangutan]

Trillian test result (tid-2320)
Environment: kvm-centos7 (x2), Advanced Networking with Mgmt server 7
Total time taken: 34390 seconds
Marvin logs: https://github.com/blueorangutan/acs-prs/releases/download/trillian/pr5551-t2320-kvm-centos7.zip
Smoke tests completed. 91 look OK, 0 have errors
Only failed tests results shown below:

Test	Result	Time (s)	Test File

[vladimirpetrov]

LGTM based on manual testing using both UI and cloudmonkey.