applyAcL should allow empty response

The method applyACL in the domain model returns the list of effective ACLs after applying this method.

The current schema definition does not allow an empty list as return value. However this method can result in the situation that the caller no longer has permission to view the ACLs and can not respond with the resulting ACL.

<xs:element name="applyACLResponse">
<xs:complexType>
<xs:sequence>
<xs:element name="ACL" type="cmism:cmisACLType"
minOccurs="1" maxOccurs="1" />
</xs:sequence>
</xs:complexType>
</xs:element>