support for "empty" components that do not participate in the coupling

In this case the GUI writes an empty transient_grid_refs element.

input/coupling.xsd.in

@@ -85,7 +85,7 @@
             <xs:complexType>
               <xs:sequence>
                 <xs:element name="transient_grid_ref" type="transient_grid_ref_type" 
-                    minOccurs="1" maxOccurs="unbounded" />
+                    minOccurs="0" maxOccurs="unbounded" />
               </xs:sequence>
             </xs:complexType>
           </xs:element>