Fix behavior of binary mutex when not applicable to inputs

scheduler/src/main/java/gov/nasa/jpl/aerie/scheduler/constraints/scheduling/BinaryMutexConstraint.java

@@ -42,11 +42,11 @@ public Windows findWindows(Plan plan, Windows windows, Conflict conflict, Simula
 
 
   private Windows findWindows(Plan plan, Windows windows, ActivityType actToBeScheduled, SimulationResults simulationResults) {
-
+    Windows validWindows = new Windows(windows);
     if (!(actToBeScheduled.equals(actType) || actToBeScheduled.equals(otherActType))) {
-      throw new IllegalArgumentException("Activity type must be one of the mutexed types");
+      //not concerned by this constraint
+      return validWindows;
     }
-    Windows validWindows = new Windows(windows);
     ActivityType actToBeSearched = actToBeScheduled.equals(actType) ? otherActType : actType;
     final var actSearch = new ActivityExpression.Builder()
         .ofType(actToBeSearched).build();