merge 回调返回 null，为什么整条记录消失了？

import java.util.HashMap;
import java.util.Map;

public class Main {
    static void check(boolean condition, String message) {
        if (!condition) throw new AssertionError(message);
    }

    static void run(String mode, boolean hasKey, Integer oldValue,
                    Integer incoming, String action) {
        Map<String, Integer> map = new HashMap<>();
        if (hasKey) map.put("item", oldValue);
        int[] calls = {0};
        Integer returned = null;
        String exception = "none";
        try {
            returned = map.merge("item", incoming, (old, added) -> {
                calls[0]++;
                return switch (action) {
                    case "remove" -> null;
                    case "keep" -> old;
                    default -> old + added;
                };
            });
        } catch (NullPointerException error) {
            exception = error.getClass().getSimpleName();
        }
        if (incoming == null) {
            check(exception.equals("NullPointerException") && calls[0] == 0,
                  "null incoming value must fail before callback");
            check(map.get("item").equals(oldValue), "old value must remain");
        } else if (oldValue == null) {
            check(calls[0] == 0 && map.get("item").equals(incoming),
                  "absent or null old value must use incoming directly");
        } else if (action.equals("remove")) {
            check(calls[0] == 1 && returned == null && !map.containsKey("item"),
                  "null callback result must remove mapping");
        } else {
            int expected = action.equals("keep") ? oldValue : oldValue + incoming;
            check(calls[0] == 1 && returned == expected && map.get("item") == expected,
                  "non-null callback result must become the value");
        }
        System.out.printf("{\"mode\":\"%s\",\"beforeHas\":%s,\"beforeValue\":%s,"
                + "\"incoming\":%s,\"action\":\"%s\",\"calls\":%d,\"returned\":%s,"
                + "\"afterHas\":%s,\"afterValue\":%s,\"exception\":\"%s\"}%n",
                mode, hasKey, oldValue, incoming, action, calls[0], returned,
                map.containsKey("item"), map.get("item"), exception);
    }

    public static void main(String[] args) {
        run("absent", false, null, 1, "remove");
        run("present-null", true, null, 1, "remove");
        run("remove", true, 2, 1, "remove");
        run("keep", true, 2, 1, "keep");
        run("add", true, 2, 1, "add");
        run("incoming-null", true, 2, null, "add");
    }
}