XML Schema : not empty simple type

Hi dear readers,

I'd like to specify the following with an xml schema :

- a simple type XXX with the constraint that its content cannot be empty.

It is possible to use a minLength facet :

<simpleType name='XXX'>
    <restriction base='string'>
        <minLength value='1'/>
    </restriction>
</simpleType>

But, does exist something like a "not null" specification for a simpleType ?

Moreover, in this case, is an element with all whitespace characters a valid element ?

Thank you for your answer.

JF