AxiomSubjectProviderEx does not honor SWRL rules with empty head

[RalphBln]

The following line does not check for possible emptiness of rule.head():

owlapi/api/src/main/java/org/semanticweb/owlapi/util/AxiomSubjectProviderEx.java

Line 245 in e0e9ef7

return rule.head().iterator().next();

In the presence of a rule with an empty head an uncaught NoSuchElementException is thrown. As one of the noticeable consequences Protégé's search window stops working completely without an error message.