title: Schema to validate null entries
body: <p>I am new to yml.</p>
  <p>My yml data file may or may not contain names under &quot;reference:&quot; tag in it. Basically, it could just be empty, So for successful validation I made it <code>'required: no'</code></p>
  <pre><code>type: map
  mapping:
    file:
      type: seq
      sequence:
        - type: map
          mapping:
            project:
              type: str
              required: yes
            sub:
              type: str
              required: yes
            source:
              type: str
              required: yes
            reference:
              type: seq
              required: no
              sequence:
                - type: map
                  mapping:
                    name:
                      type: str
                      required: no
                      unique: yes
  </code></pre>
  <p>When I try to validate yml using <code>Kwalify.pm</code> , it fails with issue:-</p>
  <pre><code> warning: Use of uninitialized value $data in concatenation (.) or string at /home/nv/utils/MFT/perl/lib/Kwalify-1.22/lib/Kwalify.pm
  </code></pre>
  <p>Need yml experts help in this. Thanks!</p>