Update dns zone name validation logic to include null check.

[imprateeksh]

Issue details:

• It appears that a validation logic for DNS zone is not correctly implemented as it missed the condition to allow null values.
• This issue was observed when consumed the version v7.20.0 in SLZ PR.

Logs:
Can check the logs here

Error image:

[imprateeksh]

cc: @SarikaSinha

[imprateeksh]

Raised PR to address this - #918

[imprateeksh]

PR merged, hence closing this issue.