request #6586: Do not display unknown user in TV3 assigned_to field

 * Suspended user selected is correctly displayed
 * Deleted user selected is displayed as None

Change-Id: I9c877cec8f2265b8729da2d33c48275b3fec7175
Merge commit 'refs/changes/14/2214/1' of http://gerrit.tuleap.net/tuleap into HEAD

* http://gerrit.tuleap.net/tuleap:
  request #6586: Do not display unknown user in TV3 assigned_to field

Change-Id: I2d015a89d2e2f3d5c1a8ff71dabee42bfaac7512
This is Tuleap 6.11.99.52

Change-Id: I7956cf3e7ddee9054c2dbe3817f592cd1dc24891