fix has and use undefined instead of null

[Mara-Li]

As discussed on the discord:

1. Before 6.1.5, the library return undefined when a value was not found/present. I created my bots (I use enmap on each! Great works!) around this because I use a lot optional (x?: string) that leads to strictly undefined. Typescript doesn't allow to set undefined = null like in Javascript. It leads to a breaking change in my base so... I changed the value returned.
2. has doesn't work in 6.1.5 and path was not accepted:

import Enmap from "enmap";

interface MyServerData {
    language: string;
    welcomeMessage: string;
    otherSettings?: {
        moderator: string;
        notificationsEnabled: boolean;
    }
}

const database: Enmap<MyServerData> = new Enmap({
    name: "myServerData",
});

database.set("server123", {
    language: "en",
    welcomeMessage: "Welcome to the server!",
    otherSettings: {
        moderator: "adminUser",
        notificationsEnabled: true,
    }
})

database.set("server123", "fr", "language");
const key = database.get("server123", "language");
const otherSettings = database.get("server123", "otherSettings.moderator");
const welcomeMessage = database.get("server123", "welcomeMessage");
const k = database.has("server123.welcomeMessage");
console.log(`Welcome Message: ${welcomeMessage} | Present: ${k}`);

Result: Welcome Message: Welcome to the server! | Present: false

[miichom]

I noticed the the has test method is missing the path, here's something I quickly did up.

describe('has', () => {
  const enmap = new Enmap({ inMemory: true });
  
  test('should return true if key exists', () => {
    enmap.set('has', 'value');
    
    expect(enmap.has('has')).toBe(true);
  });
  
+  test('should return true if key and path exists', () => {
+    enmap.set('hasPath', 'value', 'nested');
+    
+    expect(enmap.has('hasPath', 'nested')).toBe(true);
+  });
  
  test("should return false if key doesn't exist", () => {
    expect(enmap.has('unknown')).toBe(false);
  });
});