title: Render 'no content' component when section in SectionList component is empty
body: <p>See <a href="https://facebook.github.io/react-native/docs/sectionlist.html" rel="noreferrer">https://facebook.github.io/react-native/docs/sectionlist.html</a></p>
  
  <p>When one of the Section's is empty (e.a. the data prop is an empty array), I would still like to render the SectionHeader, but also render a component that indicates that the section contains no data.</p>
  
  <p>I know for the FlatList component this is possible using the ListEmptyComponent prop, but how does this work for the SectionList component?</p>
  
  <p>I was hoping for something like this (but it doesn't work): </p>
  
  <pre><code>&lt;SectionList
      style={styles.container}
      renderSectionHeader={({section}) =&gt; &lt;SectionHeader title={section.title} /&gt;}
      sections={[
        {
          data: Object.values(this.props.teams),
          title: 'Teams',
          renderItem: this._renderTeamItem,
          keyExtractor: item =&gt; item.id,
          ListEmptyComponent: this._renderEmpty
        },
        {
          data: Object.values(this.props.invites),
          title: 'Invites',
          renderItem: this._renderInviteItem,
          keyExtractor: item =&gt; item.id,
          ListEmptyComponent: this._renderEmpty
        }
      ]}
    /&gt;
  </code></pre>