Aliases: Throw exception if index is null when creating alias

Fixes a bug where alias creation would allow `null` for index name, which thereby
applied the alias to _all_ indices.  This patch makes the validator throw an
exception if the index is null.

```bash
POST /_aliases
{
   "actions": [
      {
         "add": {
            "alias": "empty-alias",
            "index": null
         }
      }
   ]
}
```
```json
{
   "error": "ActionRequestValidationException[Validation Failed: 1: Alias action [add]: [index] may not be null;]",
   "status": 400
}
```

The reason this bug wasn't caught by the existing tests is because
the old test for nullness only validated against a cluster which had
zero indices.  The null index is translated into "_all", and since
there are no indices, this fails because the index doesn't exist.
 So the test passes.

However, as soon as you add an index, "_all" resolves and you get the
situation described in the original bug report:  null index is
accepted by the alias, resolves to "_all" and gets applied to everything.

Fixes #7976