Merge branch '5.x' of github.com:terrier-org/terrier-core into 5.x

Author: cmacdonald

modules/batch-indexers/src/main/java/org/terrier/structures/indexing/classical/BasicIndexer.java

@@ -247,9 +247,9 @@ public void createDirectIndex(Collection[] collections)
 
 				Document doc = collection.getDocument();
 
-				if (doc == null)
-					continue;
-				
+				if (doc == null) {
+					logger.warn("skipping null document"); continue;
+				}
 				numberOfDocuments++; 
 				/* setup for parsing */
 				createDocumentPostings();