RM-5481 Handle empty string as a reason or exemption id.

If the empty string is passed as a classification reason or exemption id then the API framework
converts it to null for us. We should throw a 404 (not found) error, because there are no reasons
or exemptions with the empty string as their id (or null for that matter).  Add two new REST API
tests for these cases.

Also stopped using the helper method addParam in TopicsWithInstructionsTest, as it is only saving us
about ten characters, causes an extra import and includes an explicit mention of the topicsApi
inside it (i.e. it doesn't work at all with the guidesAPI).

rm-community/rm-community-repo/source/java/org/alfresco/module/org_alfresco_module_rm/util/RMCollectionUtils.java

@@ -37,6 +37,8 @@ import java.util.List;
 import java.util.Map;
 import java.util.Set;
 
+import com.google.common.collect.ImmutableSet;
+
 /**
  * Various common helper methods for Collections. This class is probably only appropriate for use with relatively
  * small collections as it has not been optimised for dealing with large collections.
@@ -201,4 +203,27 @@ public final class RMCollectionUtils
             else                     { return Difference.UNCHANGED; }
         }
     }
+
+    /**
+     * Convert a collection to an immutable set. Any instances of null in the original collection will be removed.
+     *
+     * @param collection The original collection.
+     * @param <T> The type of the object in the collection.
+     * @return The immutable set.
+     */
+    public static <T> ImmutableSet<T> toImmutableSet(Collection<T> collection)
+    {
+        if (collection == null)
+        {
+            return null;
+        }
+        // Guava immutable collections can't contain null.
+        if (collection.contains(null))
+        {
+            // Make sure we're not changing the original collection (which might not be editable anyway).
+            collection = new HashSet<>(collection);
+            collection.remove(null);
+        }
+        return ImmutableSet.copyOf(collection);
+    }
 }