Allow the IAutoIDGenerator to be null.

Amend the AutoIDAllocator class to allow it to be used with a null
IAutoIDGenerator -- in which cases it obviously never allocates any ID.

This makes it easier to ensure that we can never let IDs in the auto-ID
namespace (https://w3id.org/kgcl/autoid/) pass into the target ontology
without being re-assigned, in the event we cannot obtain a suitable ID
generator -- all we have to do is check the return value of the
reallocate() method, which will return false if we have not been able to
reassign any ID.

core/src/main/java/org/incenp/obofoundry/kgcl/AutoIDAllocator.java

@@ -89,7 +89,9 @@ private boolean isAuto(String id) {
     private String getAutoID(String id) {
         String autoID = idMap.get(id);
         if ( autoID == null ) {
-            autoID = idGenerator.nextID();
+            if ( idGenerator != null ) {
+                autoID = idGenerator.nextID();
+            }
             if ( autoID == null ) {
                 unallocatedIDs.add(id);
                 autoID = id;