minSize constraint validates is true when null value is present

- [X] Steps to reproduce provided
- [ ] Stacktrace (if present) provided
- [X] Example that reproduces the problem uploaded to Github
- [X] Full description of the issue provided (see below)

### Steps to Reproduce

These steps are true wether using domain objects or general validated objects

If you have a parent class and child class as follows
```
class Person implements Validateable {
String name

static constraints = {
name nullable: false
}
}

class Employer extends Person{
String company
List employees

static constraints = {
company nullable: false
employees minSize: 5
}
}
```

If you call `.validate()` on an this `Employer child = new Employer(name: 'TEST', company: 'TEST')` this will validate as true even when employees is null.

If you call `.validate()` on an this ```Employer child = new Employer(name: 'TEST', company: 'TEST')
child.employees = [employee1]``` this will validate as false as employees are less than 5.

If you call `.validate()` on an this ```Employer child = new Employer(name: 'TEST', company: 'TEST')
child.employees = [employee1, employee2, employee3, employee4, employee5]``` this well validate as true.

### Expected Behaviour

That when employees is null, validation fails due to the minimum number not being met.

### Actual Behaviour

That when employees is null, validation passes.

Note: if you want to produce expected behaviour it is possible by adding `nullable: false`
`employees nullable:false, minSize: 5`

### Environment Information

- **Operating System: MacOS 10.13.3
- **GORM Version: 6.1.9.RELEASE
- **Grails Version (if using Grails): 3.3.4
- **JDK Version: openjdk version "1.8.0_163"

### Example Application

https://github.com/benrhine/validateableInheritence

You can see the issue in EmployerValidationIntSpec, which uses Person and Employer objects

Start with EmployerValidationIntSpec in the linked validateableInheritence example application and reproduce the null employees case. Trace the validation behavior for the minSize: 5 constraint, then add regression coverage showing that null fails while five employees passes, and run the relevant specification.

Written by the indexing model from the issue text.