[BUGFIX] Show owner only if owner exists

If the fe_users record does not exists, the owner entry in the
extension single view is skipped.

Resolves: #60557
Change-Id: I0c4ab23f712d160078289f027cafc69c95ee8406
Reviewed-on: http://review.typo3.org/31800
Reviewed-by: Tomas Norre Mikkelsen <tomasnorre@gmail.com>
Tested-by: Tomas Norre Mikkelsen <tomasnorre@gmail.com>