Describe the feature or problem you’d like to solve

We use the new github projects to keep track of custom fields. Some fields only make sense for some issue states. We would like to build automation based on gh to remove the field values when the issue state changes.

gh project item-edit already allows to set the field value (good) but it does not support removing the value of fields (bad, see #7910).

Proposed solution 1

Allow to pass an empty string to remove the value of the field:

gh project item-edit ... --number ''
gh project item-edit ... --single-select-option-id ''

Proposed solution 2

Support an explicit option to remove the value of the field:

gh project item-edit ... --clear

(I used to propose --empty but a comment below points out that the GraphQL interaction is called clear so that is a better name also for the CLI).