How can I go about a) permitting null values for an element defined in an XSD file, and b) express a null value in an XML element?

I note the "nillable" attribute for an element definition, but setting the nillable attribute to "true" doesn't allow me to either leave the element empty, or with a value of "NULL" or "nil".

FWIW, the element I'm dealing with uses a float data type, but needs to permit the equivalent of a null value.

Thanks.