[bugfix] Avoid NPE in RESTXQ when extracting empty request body

Avoid an NPE in RESTXQ when there is no request body when using `%rest:POST` and/or `%rest:PUT` annotations to extract the body e.g.

```xquery
xquery version "3.0";

declare
    %rest:GET
    %rest:POST("{$body}")
    %rest:PUT("{$body}")
    %rest:DELETE
    %rest:path("/empty-body")
function local:get($body) {
    <body>{$body}</body>
};

()
```

```bash
curl -v -X POST http://admin:@localhost:8080/exist/restxq/empty-body
```