title: XmlPeek empty string causes failure
body: <p>So in my targets file, I've got a line that looks like this:</p>
  
  <pre><code>&lt;XmlPeek Namespaces="" XmlInputPath="file.xml" Query="/data/@AttributeOne"&gt;
    &lt;Output TaskParameter="Result" ItemName="my_AttributeOne" /&gt;
  &lt;/XmlPeek&gt;
  </code></pre>
  
  <p>in "file.xml", I have:</p>
  
  <pre><code>&lt;data AttributeOne="abc" AttributeTwo="def" /&gt;
  </code></pre>
  
  <p>it also reads a few other attributes.</p>
  
  <p>When the attribute has data, everything works fine... but when I leave AttributeOne as an empty string (""), XmlPeek blows chunks with the following error:</p>
  
  <blockquote>
    <p>The "XmlPeek" task's outputs could not be retrieved from the "Result" parameter. Parameter "includeEscaped" cannot have zero length.</p>
  </blockquote>
  
  <p>if I remove the attribute ENTIRELY, it works fine (the resulting item is obviously and understandably blank)</p>
  
  <p>The question is... how can I DETERMINE, WITHOUT blowing chunks, the value of a blank attribute... whether by pre-testing for a value, or by correctly handling the blank, or some other means.</p>
  
  <hr>
  
  <p>CONSTRAINT: the only real requirement is to stick to the built-in tasks (XmlPeek)... I'm aware of XmlRead in the community tasks... for various reasons, I want to use out-of-the-box tasks.</p>
  
  <p>Thanks in advance!</p>