space: document method argument as Nullable

Motivation:

SpotBugs warns that a nonnull argument may be called with null.

Modification:

Update documentation that null is valid for this method argument.

Result:

Clear one SpotBugs NORMAL level warning.

Target: master
Requires-notes: no
Requires-book: no
Patch: https://rb.dcache.org/r/12593/
Acked-by: Tigran Mkrtchyan

modules/dcache-spacemanager/src/main/java/diskCacheV111/services/space/SpaceManagerDatabase.java

@@ -64,7 +64,7 @@ long updateLinkGroup(String linkGroupName,
                          boolean replicaAllowed,
                          boolean outputAllowed,
                          boolean custodialAllowed,
-                         VOInfo[] linkGroupVOs) throws DataAccessException;
+                         @Nullable VOInfo[] linkGroupVOs) throws DataAccessException;
 
     LinkGroup getLinkGroup(long id) throws DataAccessException;