fix: correctly report NoUsers for ExamRoomRandom

preselection was ignored for the check so far

src/Handler/Exam/AutoOccurrence.hs

@@ -1,5 +1,4 @@
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
-{-# OPTIONS_GHC -Wwarn #-}
 
 module Handler.Exam.AutoOccurrence
   ( examAutoOccurrenceCalculateWidget
@@ -199,7 +198,6 @@ postEAutoOccurrenceR tid ssh csh examn = do
 
   let calculateRes' = asum $ calculateRes : nudgeRes ^.. folded . _1 . _1 ++ ignoreRes ^.. folded . _1 . _1
 
-
   calcResult <- formResultMaybe calculateRes' $ \ExamAutoOccurrenceCalculateForm{..} -> do
     let autoOccurrenceResult = examAutoOccurrence eId examOccurrenceRule eaofConfig occurrences' participants'
     (eaofMapping, eaofAssignment, eaofSuccess) <- case autoOccurrenceResult of

src/Handler/Utils/Exam.hs

@@ -407,8 +407,10 @@ examAutoOccurrence (hash -> seed) rule ExamAutoOccurrenceConfig{..} occurrences
                    ]
                takeEnd n chars = drop (length chars - n) chars
             in Map.mapKeysWith Set.union (takeEnd . F.minimum . Set.map length $ Map.keysSet matrUsers) matrUsers
-      _ | null users-> Map.empty
-        | otherwise -> Map.singleton [] $ Map.keysSet users
+      _ | null unassignedUsers -> Map.empty
+        | otherwise -> Map.singleton [] $ Map.keysSet unassignedUsers
+      where
+        unassignedUsers = Map.filter (has $ _2 . _Nothing) users
 
     occurrencesSize :: ExamOccurrenceCapacity
     occurrencesSize = foldMap' (view _2) occurrences''