Replace assertionFailure with debugPrint to stop assertions while…

… debugging if a nil action block was passed in on purpose. (#2005)

Sources/Apollo/DispatchQueue+Optional.swift

@@ -27,7 +27,7 @@ public extension ApolloExtension where Base == DispatchQueue {
         action(result)
       }
     } else if case .failure(let error) = result {
-      assertionFailure("Encountered failure result, but no completion handler was defined to handle it: \(error)")
+      debugPrint("Apollo: Encountered failure result, but no completion handler was defined to handle it: \(error)")
     }
   }
 }