Steps to reproduce

1. Create a model with a property of either of the following types: JSON, Any, or Object
2. Ensure the property above is not required.
3. Create an instance of the above model setting the property value to null
4. Query the model where the value of the property is null and see that the created instance is not returned

Current Behavior

'null' is stored in the dataSource

Expected Behavior

null is stored in the dataSource

Proposed fix

Replace this:

    if (type === 'JSON' || type === 'Any' || type === 'Object' || data[propName] instanceof Array) {
      res[propName] = JSON.stringify(data[propName]);
    } else {
      res[propName] = data[propName];
    }

With this:

    const value = data[propName];
    if (value !== null && (type === 'JSON' || type === 'Any' || 
        type === 'Object' || value instanceof Array)) {
      res[propName] = JSON.stringify(value);
    } else {
      res[propName] = value;
    }

Additional information

win32 x64 12.18.3
"loopback": "^3.28.0",
"loopback-datasource-juggler": "^3.28.0",

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