Querying for an attribute that is not available in the document will indeed make value() return NULL, just like when an element is not specified. However, not specifying the attribute may have different interpretation than specifying an element with a nil attribute, even though in both situations the value() function will return NULL.
using xpath's not() function makes null handling really simple, especially in xsl transforms. //Employee[not(@certification)] will easily find all Employee elements with no certification attribute (i.e.: Employee.certification is null).