Graphql on a @GrailsCompileStatic domain class cause nullable constraint to be ignored

If you have a domain class that is marked with @GrailsCompileSatic and this class has a property with a constraint nullable: true, and if this class is enabled for GraphQL with a custom operation/query, then the nullable: true constraint behaves like nullable: false.

Steps to reproduce:
1. Create a new project (Tested with Grails 4.0.4 / 4.0.5 and their default gorm version)
Add `compile "org.grails.plugins:gorm-graphql-plugin:2.0.0"` dependency

2. Create a domain class, say `MyDomain` marked with `@GrailsCompileStatic` and a `nullable` property, like:
```
@GrailsCompileStatic
class MyDomain {
String name

static constraints = {
name nullable: true
}
}
```

3. Add a custom GraphQL mapping which defines a simple query:
```
@GrailsCompileStatic
class MyDomain {
String name

static constraints = {
name nullable: true
}

static graphql = GraphQLMapping.build {
query("myQuery", Integer) {
dataFetcher(new DataFetcher() {
@Override
Object get(DataFetchingEnvironment environment) throws Exception {
return new Integer(0)
}
})
}
}
}
```

_If you just put `static graphql=true` or `static graphql = GraphQLMapping.build {}` there's no issue._

4. Now in BootStrap, you just create and save a new instance of that class leaving the nullable field empty, like:
```
class BootStrap {

def init = { servletContext ->
new MyDomain().save(failOnError: true)
}
def destroy = {
}
}
```

5. Now you run with `gradlew bootRun`. It is expected the instance to be persisted with no errors, because it has only one field, which is `null`, and that field allows `null`. Instead, you see a validation exception:
```
grails.validation.ValidationException: Validation Error(s) occurred during save():
- Field error in object 'test.MyDomain' on field 'name': rejected value [null]; codes [test.MyDomain.name.nullable.error.test.MyDomain.name,test.MyDomain.name.null
able.error.name,test.MyDomain.name.nullable.error.java.lang.String,test.MyDomain.name.nullable.error,myDomain.name.nullable.error.test.MyDomain.name,myDomain.name.
nullable.error.name,myDomain.name.nullable.error.java.lang.String,myDomain.name.nullable.error,test.MyDomain.name.nullable.test.MyDomain.name,test.MyDomain.name.nu
llable.name,test.MyDomain.name.nullable.java.lang.String,test.MyDomain.name.nullable,myDomain.name.nullable.test.MyDomain.name,myDomain.name.nullable.name,myDomain
.name.nullable.java.lang.String,myDomain.name.nullable,nullable.test.MyDomain.name,nullable.name,nullable.java.lang.String,nullable]; arguments [name,class test.My
Domain]; default message [O campo [{0}] da classe [{1}] nÒo pode ser vazio]

at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
at org.codehaus.groovy.reflection.CachedConstructor.invoke(CachedConstructor.java:80)
at org.codehaus.groovy.reflection.CachedConstructor.doConstructorInvoke(CachedConstructor.java:74)
at groovy.lang.MetaClassImpl.invokeConstructor(MetaClassImpl.java:1732)
at groovy.lang.MetaClassImpl.invokeConstructor(MetaClassImpl.java:1556)
at org.codehaus.groovy.runtime.InvokerHelper.invokeConstructorOf(InvokerHelper.java:1042)
at org.codehaus.groovy.runtime.DefaultGroovyMethods.newInstance(DefaultGroovyMethods.java:17159)
at org.grails.orm.hibernate.AbstractHibernateGormInstanceApi.save(AbstractHibernateGormInstanceApi.groovy:134)
at org.grails.datastore.gorm.GormEntity$Trait$Helper.save(GormEntity.groovy:153)
at test.MyDomain.save(MyDomain.groovy)
at test.MyDomain.save(MyDomain.groovy)
at org.grails.datastore.gorm.GormEntity$save.call(Unknown Source)
at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:47)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:115)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:127)
at testconstraintnotnull.BootStrap$_closure1.doCall(BootStrap.groovy:10)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.codehaus.groovy.reflection.CachedMethod.invoke(CachedMethod.java:101)
at groovy.lang.MetaMethod.doMethodInvoke(MetaMethod.java:323)
at org.codehaus.groovy.runtime.metaclass.ClosureMetaClass.invokeMethod(ClosureMetaClass.java:263)
at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:1041)
at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:1099)
at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:1041)
at groovy.lang.Closure.call(Closure.java:405)
at groovy.lang.Closure.call(Closure.java:399)
at grails.util.Environment.evaluateEnvironmentSpecificBlock(Environment.java:594)
at grails.util.Environment.executeForEnvironment(Environment.java:587)
at grails.util.Environment.executeForCurrentEnvironment(Environment.java:563)
at org.grails.web.servlet.boostrap.DefaultGrailsBootstrapClass.callInit(DefaultGrailsBootstrapClass.java:74)
at org.grails.web.servlet.context.GrailsConfigUtils.executeGrailsBootstraps(GrailsConfigUtils.java:83)
at org.grails.plugins.web.servlet.context.BootStrapClassRunner.onStartup(BootStrapClassRunner.groovy:56)
at grails.boot.config.GrailsApplicationPostProcessor.onApplicationEvent(GrailsApplicationPostProcessor.groovy:269)
at grails.boot.config.GrailsApplicationPostProcessor.onApplicationEvent(GrailsApplicationPostProcessor.groovy)
at org.springframework.context.event.SimpleApplicationEventMulticaster.doInvokeListener(SimpleApplicationEventMulticaster.java:172)
at org.springframework.context.event.SimpleApplicationEventMulticaster.invokeListener(SimpleApplicationEventMulticaster.java:165)
at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:139)
at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:403)
at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:360)
at org.springframework.context.support.AbstractApplicationContext.finishRefresh(AbstractApplicationContext.java:897)
at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.finishRefresh(ServletWebServerApplicationContext.java:162)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:553)
at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:141)
at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:744)
at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:391)
at org.springframework.boot.SpringApplication.run(SpringApplication.java:312)
at grails.boot.GrailsApp.run(GrailsApp.groovy:99)
at grails.boot.GrailsApp.run(GrailsApp.groovy:485)
at grails.boot.GrailsApp.run(GrailsApp.groovy:472)
at testconstraintnotnull.Application.main(Application.groovy:11)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.springframework.boot.devtools.restart.RestartLauncher.run(RestartLauncher.java:49)
```

Now if you either remove @GrailsCompileStatic or remove the custom query or remove `graphql` at all, the app just runs

I uploaded a semple [here](https://github.com/magnomp/TestConstraintNotNull)