Why in Collectors.toMap(keyMapper, valueMapper, mergeFunction, mapSupplier), valueMapper param not allowed null?

Question

I have this block of code to create a deep copy of a Map<String, Object>:

    Map<String, Object> originalMap = new HashMap<>();
    originalMap.put("key1", "value1");
    originalMap.put("key2", null);
    originalMap.put("key3", "value3");

    Map<String, Object> newMap = originalMap.entrySet()
        .stream()
        .collect(Collectors.toMap(
            Map.Entry::getKey,
            // Map.Entry::getValue,
            // entry -> entry.getValue() != null ? entry.getValue() : null,
            entry -> entry.getValue() != null ? entry.getValue() : "",
            (oldValue, newValue) -> oldValue,
            HashMap::new
        ));

    System.out.println("Original map: " + originalMap);
    System.out.println("New map: " + newMap);

If I use 2 commented line, this codeblock throw Null Pointer Exception and I don't understand this. I know that HashMap does allow Null values so why it's not accept the value mapper to return null?