title: Unity resolving unmapped types with null
body: <p>If I don't have the concrete class mapped to the interface, when Unity tries to resolve the type it gives me this error: "<em>The current type, IFoo, is an interface and cannot be constructed. Are you missing a type mapping?</em>".</p>
  
  <p>However, for <strong>testing purposes</strong>, I'd like Unity to pass null to the interfaces that aren't mapped yet to concrete types.</p>
  
  <p>Any suggestion to make this as the default behavior to "resolve" unmapped interfaces?</p>
  
  <p>Thanks</p>