Change: Allow for empty variable values

[CGoodwin90]

Updates the Variables tabs to allow for variables to be created/updated with no value defined. Includes confirm pop-up when adding/updating variable to have a null value.

Displays empty variables with a Tag component:

[tobybellwood]

This looks good for entering, but we need to consider how to display an empty var - having a dash may be confusing, but not sure what the alternatives are

[CGoodwin90]

This looks good for entering, but we need to consider how to display an empty var - having a dash may be confusing, but not sure what the alternatives are

Pulled in the Tag component from antd, thoughts?