Avoid requiring non-existent fields when using Grape::Entity documentation

When `requires :none, except: [:non_existent_field], using: documentation`, it may generate the params like this

```
{
    "paramType": "form",
    "name": "non_existent_field",
    "description": null,
    "type": "String",
    "dataType": "String",
    "required": true
},
{
    "paramType": "path",
    "name": "",
    "description": null,
    "type": "String",
    "dataType": "String",
    "required": true
},
{
    "paramType": "form",
    "name": "required_field",
    "description": null,
    "type": "String",
    "dataType": "String",
    "required": false
}
```