title: How to achieve a valid null-option with select ng-options
body: <p>There are a couple of questions and answers around this topic here, but I cannot find a solution which is working the way it should be for my case.</p>
  
  <p>Just imagine I have an object like this</p>
  
  <pre><code>$scope.person = {name: 'Peter', category1: null, category2: null};
  </code></pre>
  
  <p>In a different variable I receive a list of categories via a <code>$resource</code> call with a result like this:</p>
  
  <pre><code>$scope.categories = [{id:1, name:'Supplier'}, {id:2, name:'Customer'}];
  </code></pre>
  
  <p>Now it's easy to build a select with <code>ng-options</code> to choose from <code>categories</code> to set the selected <code>category.id</code> as <code>person.category1</code> or <code>person.category2</code>.</p>
  
  <p>But how can I do that when <code>category1</code> is mandatory while <code>category2</code> can still be a valid <em>null</em> value?</p>
  
  <p>So basically what I am looking for now are two selects with the following options:</p>
  
  <pre><code>//Select1
  - Select Category1 (disabled)
  - Customer (value: 1)
  - Supplier (value: 2)
  
  //Select2
  - Select Category2 (disabled)
  - No Category (value: null)
  - Customer (value: 1)
  - Supplier (value: 2)
  </code></pre>
  
  <h2>EDIT</h2>
  
  <p>I added a <a href="http://plnkr.co/edit/F3R3ajt4Uwxawq0JVIaC?p=preview" rel="nofollow noreferrer"><strong>Plunkr</strong></a> based on <a href="https://stackoverflow.com/a/42342420/4927984"><em>@Mistalis answer</em></a>, which shows what I want to achieve: Each select should have a disabled placeholder option and one should support a "valid null option".</p>