resilience: do not return null if tag map is null

Motivation:

The expected result of the call to PoolInfoMap.getTags() is an
empty map even for uninitialized pools, not null.  The case
where there is no pool information entry was being handled
correctly, but not the case where the information object
was not yet entirely initialized.   This was causing an NPE
in one of the handler methods.

Modification:

Make the method return empty map in the latter case as well.

Result:

No NPE.

modules/dcache/src/main/java/org/dcache/resilience/data/PoolInfoMap.java

@@ -554,7 +554,13 @@ public Map<String, String> getTags(Integer pool) {
         if (info == null) {
             return ImmutableMap.of();
         }
-        return info.getTags();
+
+        Map<String, String> tags = info.getTags();
+        if (tags == null) {
+            return ImmutableMap.of();
+        }
+
+        return tags;
     }
 
     public Set<Integer> getValidLocations(Collection<Integer> locations,

modules/dcache/src/main/java/org/dcache/resilience/data/PoolOperationMap.java

@@ -678,7 +678,7 @@ public void update(String pool, int children) {
      * <p>Called by the {@link PnfsOperationMap) when a child operation completes.</p>
      */
     public void update(String pool, PnfsId pnfsId) {
-        LOGGER.trace("Parent {}, child operation for {} has canceled.", pool,
+        LOGGER.trace("Parent {}, child operation for {} has completed.", pool,
                      pnfsId);
         lock.lock();
         try {