Ability to provide null/void value in env variable override

### Before opening, please confirm:

- [X] I have checked to see if my question is addressed in the [FAQ](https://github.com/aws-amplify/amplify-hosting/blob/master/FAQ.md).
- [X] I have [searched for duplicate or closed issues](https://github.com/aws-amplify/amplify-hosting/issues?q=is%3Aissue+).
- [X] I have removed any sensitive information from my code snippets and submission.

### Amplify Hosting feature

Environment variables

### Is your feature request related to a problem? Please describe:

I would like to raise a feature request to provide void value for a particular environmental variable. This is not possible in Amplify Console UI because the value field is required. Similar to the below Github issues :-

[+] https://github.com/aws-amplify/amplify-hosting/issues/1202

### Describe how you'd like this feature to work

I would like the ability to provide null value in env variable override using Amplify Console UI.

Start with the Amplify Console UI environment-variable override flow and review the related issue #1202 for prior context. Confirm how the required value field currently handles overrides, then define completion as allowing an explicitly null or void override while preserving existing non-null values.

Written by the indexing model from the issue text.