Unique constraints on blank field is not checked

Consider the domain object:

``` groovy
class Cookie {
String name
static constraints = {
name nullable: false, blank: true, unique: true
}
}
```

Then the following test passes although it shouldn't.

``` groovy
class CookieSpec extends IntegrationSpec {
def 'validation fails'() {
setup:
def cookie = new Cookie()
def sameCookie = new Cookie()
cookie.name = ''
sameCookie.name = ''
Cookie.withTransaction{
cookie.save()
}
expect:
sameCookie.validate()
}
}
```

One cannot set the name to the empty string in the Cookie constructor as it would then yield null due to the data binding mechanism.
## Note

Not checking the unique constraint is the correct behavior for the null value, see https://jira.grails.org/browse/GRAILS-10403. At least for MySQL databases forcing the empty string to be a unique value would be consistent with the database behavior.
## Workaround

One can use a custom validator, which in our example would be

``` groovy
class Cookie {
String name
static constraints = {
name nullable: false, blank: true, validator: { val, obj -> ! Cookie.findByName(obj.name) }
}
}
```

Start by reproducing the behavior in the CookieSpec IntegrationSpec example with two Cookie objects whose name is blank, then trace the validation path for the unique constraint. Compare the handling of blank and null values, including the noted MySQL behavior and the GRAILS-10403 reference. Done means the expected blank-value uniqueness behavior is established and covered by a regression test.

Written by the indexing model from the issue text.