How can I check if a vector with a custom class is NULL?

I have made this custom class

#ifndef VEHICLE_H_
#define VEHICLE_H_

Class Vehicle {
 public:
 Vehicle();
 Vehicle(char,char,int,int);
 virtual ~Vehicle();
 char getLicense_plate();
 void setLicense_plate(char);
 char getBrand();
 void setBrand(char);
 int getTime_in();
 void setTime_in(int);
 int getTime_out();
 void setTime_out(int);

 char license_plate;
 char brand;
 int timei;
 int timeo;
};

And I have created a Vehicle vector with size 50 in the main.cpp but I don't how to check each value if they are empty.

#inlcude<iostream>
#include<vector>
#include<algorithm>
#inlude "Vehicle.h"
using namespace std;

int main()
{
vector<Vehicle> avai_space(50);
for(int i=0;i<avai_space.size();i++)
 { 
  //if(avai_space(i) == NULL??){}