Fix for crash when pushable project have unknown users

Apparently we've got Unknown Object (Phabricator User) in
translation project now.

Not sure how it became invalid, but automated scripts
better be robust for this.

scripts/gitadmin/rebuild_gitadmin.php

@@ -41,6 +41,9 @@ function handleSingleUserPHID(
     ->setViewer($viewer)
     ->withPHIDs(array($userPHID))
     ->executeOne();
+  if (!$user) {
+    return;
+  }
 
   $keys = id(new PhabricatorUserSSHKey())->loadAllWhere(
    'userPHID = %s',

scripts/svnauth/rebuild_svnauth.php

@@ -17,6 +17,9 @@ function handleSingleUserPHID(
     ->setViewer($viewer)
     ->withPHIDs(array($userPHID))
     ->executeOne();
+  if (!$user) {
+    return;
+  }
 
   $user_name = $user->getUserName();
   if (!array_key_exists($user_name, $authfile)) {