title: jq filter- unexpected "null"s
body: <p>input json:</p>
  
  <pre><code>{
    "id": 381,
    "name": "Module",
    "value": "C-A",
    "locked": false
  }
  {
    "id": 381,
    "name": "Module",
    "value": "",
    "locked": false
  }
  {
    "id": 381,
    "name": "Module",
    "value": "",
    "locked": false
  }
  </code></pre>
  
  <p>And I want to remove all objects with value unset <code>""</code>
  I have such filter but it prints also <code>null</code> - how to rid it of?</p>
  
  <pre><code>$ jq 'del(select(.value == ""))' /tmp/intest
  {
    "id": 381,
    "name": "Module",
    "value": "C-A",
    "locked": false
  }
  null
  null
  </code></pre>
  
  <p>update:
  I want also to join non-empty values but get error:</p>
  
  <pre><code>$ jq 'select(.value!="")|.value|join(",")' /tmp/intest
  jq: error (at /tmp/intest:6): Cannot iterate over string ("C-A")
  </code></pre>