Namespace Drop-Down Default to Null on New Variable Creation UI

### Proposal
When creating a new variable via UI (`ui/variables/new[?path=nomad%2Fjobs%2F]`), I would like to propose two small changes:
1. the pre-selected value for the namespace drop-down menu is an empty/blank/null namespace
2. the 'Save Variables' button enforces validation of the selection from the namespace drop-down menu before saving and then does not proceed until a namespace value is explicitly chosen (thereby forcing the user to choose a namespace)

### Use-cases
Often times I'll go to create a new variable and, in doing so, I'll forget to explicitly choose a namespace for it (thereby defaulting to whatever existing namespace is alphabetically first). Then I'll create populate the new variable with its necessary kv-pairs. Then I'll perform this same process again in our federated region. Finally I'll repeat this entire process again in our dev environment.

This is when it hits me: "Blast! _::shakes fist at sky::_ I just created this var in the wrong namespace." Then I trace back my steps, delete the var (in all envs and regions) and recreate it in the proper namespace.

### Alternate Proposal
Allow for easily moving a Nomad Variable from one namespace to another.

### Attempted Solutions
Only workaround for this is for me to not be such an idiot (hard sell; trust me: I've tried 😆).

### Nomad Environment Details
Nomad v1.9.1+ent

Start at the ui/variables/new route, including the path query shown in the issue, and inspect the namespace drop-down and Save Variables flow. Done means new variables begin with no namespace selected, saving requires an explicit namespace choice, and saving does not proceed when validation fails.

Written by the indexing model from the issue text.