How to test an initialize method that returns "this" with a only one if condition to set the internal data

I try to figure out how to test this kind of method

// Let's say models === null when we instantiate
public initialize(mongodb: MongoDb): this {
    if (!this.models) {
      this.models = {
        users: new models.UserModel(mongodb),
      };
    }
    return this;
  }

  public getModels(): Models | null {
    return this.models || null;
  }

My coverage is still staying that I didn't test the if part... This is not really true because when I ask for the getModels I can test its value (so implicitly the if.

Any idea?