[test] Correct failing test.

Author: chris-evolvedbinary

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

@@ -150,7 +150,11 @@ public int getColumn() {
 
     @Override
     public Source getSource() {
-        return context.getSource();
+        try {
+            return context.getSource();
+        } catch (IllegalStateException e) {
+            return null;
+        }
     }
 
     @Override