Empty select element options are considered invalid
### Describe The Bug

Empty values are used in this project (although this might not be in use):
https://github.com/homebridge/homebridge-config-ui-x/blob/57246cf/config.schema.json#L439

And also here:
https://github.com/OpenWonderLabs/homebridge-switchbot/blob/e9bbc7d/config.schema.json#L357
<img width="50%" src="https://github.com/user-attachments/assets/4f33335c-2392-4de6-8342-56995c4862dd">

This could be fixed by replacing `hasValue` with `isDefined` here:
https://github.com/zahmo/ng-formworks/blob/32f31f507150857d530b3c667716a1ee9a8cf87c/projects/ng-formworks-core/src/lib/shared/json.validators.ts#L154

### Logs

_No response_

### Config

_No response_

### Homebridge UI Version

Latest branch

### Homebridge Version

v1.8.4

### Node.js Version

v20.11.1

### Operating System

macOS

### Environment Info

None

### Raspberry Pi Model

None