.default() does not work with Number 0 as value when used with checkSchema

[ghost]

Describe the bug

The default sanitizer should set default value to missing or null values.

Although, it doesn't work when we use it inside a schema and we want to set value to the Number 0.

To Reproduce

Simple script to reproduce:

const express = require('express');
const { body, checkSchema } = require('express-validator');

const app = express();

app.use(express.json());

app.post(
  "/schema/number",
  checkSchema({
    test: {
      in: ['body'],
      default: {options: 0}
    }
  }),
  (req, res, next) => {
    console.log(`/schema/number output: ${JSON.stringify(req.body, null, 4)}`);
    next();
  }
);

app.post(
  "/schema/string",
  checkSchema({
    test: {
      in: ['body'],
      default: {options: "0"}
    }
  }),
  (req, res, next) => {
    console.log(`/schema/string output: ${JSON.stringify(req.body, null, 4)}`);
    next();
  }
);

app.post(
  "/chain/number",
  body('test').default(0),
  (req, res, next) => {
    console.log(`/chain/number output: ${JSON.stringify(req.body, null, 4)}`);
    next();
  }
);

app.post(
  "/chain/string",
  body('test').default('0'),
  (req, res, next) => {
    console.log(`/chain/string output: ${JSON.stringify(req.body, null, 4)}`);
    next();
  }
);

app.listen(3000);

With express-validator 6.12.1 the output is:

/chain/number output: {
    "test": 0
}
/chain/string output: {
    "test": "0"
}
/schema/number output: {}
/schema/string output: {
    "test": "0"
}

Expected behavior

With same program, the expected behavior is to get 0 with the /schema/number route, as the /chain/number does:

/chain/number output: {
    "test": 0
}
/chain/string output: {
    "test": "0"
}
/schema/number output: {
    "test": 0
}
/schema/string output: {
    "test": "0"
}

Current behavior

See above.

Environment:

• Express-validator version: 6.12.1
• Express version: 4.17.1