[fix] null contextItem and ~Sequence in functions

- User defined functions do not receive contextItem and
  contextSequence according to the XQuery 3.1 specification.
- The DeferredFunctionCallTest is adapted to reflect that change.
- The method `Query#preSelect` in the lucene extension returns an empty
  set when the contextSequence is null. It would otherwise result in an
  NPE.

Author: line-o

exist-core/src/main/java/org/exist/xquery/UserDefinedFunction.java

@@ -158,7 +158,7 @@ public Sequence eval(Sequence contextSequence, Item contextItem) throws XPathExc
  						". Expected " + getSignature().getArgumentTypes()[j].getCardinality().getHumanDescription() +
  						", got " + currentArguments[j].getItemCount());}
 			}
-			result = body.eval(contextSequence, contextItem);
+			result = body.eval(null, null);
 			return result;
 		} finally {
 			// restore the local variable stack

exist-core/src/test/java/org/exist/xquery/DeferredFunctionCallTest.java

@@ -102,7 +102,7 @@ public void ensure_argumentsToDeferredFunctionCall_AreNotLost_AfterReset_And_Bef
         expect(mockContext.declareVariableBinding((LocalVariable)anyObject())).andReturn(null); 
         expect(mockFunctionSignature.getArgumentTypes()).andReturn(mockArgumentTypes); 
 
-        expect(mockExpression.eval(mockContextSequence, mockContextItem)).andReturn(Sequence.EMPTY_SEQUENCE);
+        expect(mockExpression.eval(null, null)).andReturn(Sequence.EMPTY_SEQUENCE);
 
         mockExpression.resetState(true);
 

extensions/indexes/lucene/src/main/java/org/exist/xquery/modules/lucene/Query.java

@@ -197,16 +197,17 @@ public int getOptimizeAxis() {
     }
 
     public NodeSet preSelect(Sequence contextSequence, boolean useContext) throws XPathException {
-    	if (contextSequence != null && !contextSequence.isPersistentSet())
+        // guard against an empty contextSequence
+    	if (contextSequence == null || !contextSequence.isPersistentSet()) {
     		// in-memory docs won't have an index
     		return NodeSet.EMPTY_SET;
-    	
+        }
+
         long start = System.currentTimeMillis();
         // the expression can be called multiple times, so we need to clear the previous preselectResult
         preselectResult = null;
         LuceneIndexWorker index = (LuceneIndexWorker) context.getBroker().getIndexController().getWorkerByIndexId(LuceneIndex.ID);
 
-        // DW: contextSequence can be null
         DocumentSet docs = contextSequence.getDocumentSet();
         Item key = getKey(contextSequence, null);
         List<QName> qnames = new ArrayList<>(1);