title: How to fix flow type length error in array?
body: <p>Let say my code like below</p>
  
  <pre><code>//@flow
  type something = (input:Array&lt;{data:?number}|{data&lt;Object&gt;}&gt;) =&gt; boolean
  
  function some:something(own){
   if(!own[0].data.length)
    return false
  }
  </code></pre>
  
  <p>i got message:
  property <code>length</code>,
  Property cannot be accessed on possibly null value
  null or undefined</p>
  
  <p>how to fixed that?
  because in function some can be parsing
   1. Array with inside object or maybe not object.
   2. Array with inside Object and in Object can be data with Array of Object or Array of null/undefine
   3. how declare length in flow type if like that</p>