Make the index-to-project map empty rather than null in the BWC deserialization case.

This works out fine, for the reasons given in the comment. As it happens, I'd already forgotten to do the null check in the one place it's actively used.

Author: PeteGillinElastic

server/src/main/java/org/elasticsearch/indices/NodeIndicesStats.java

@@ -94,10 +94,11 @@ public NodeIndicesStats(StreamInput in) throws IOException {
         }
 
         if (in.getTransportVersion().onOrAfter(TransportVersions.NODES_STATS_SUPPORTS_MULTI_PROJECT)) {
-            boolean hasProjectsByIndex = in.readBoolean();
-            projectsByIndex = hasProjectsByIndex ? in.readMap(Index::new, ProjectId::readFrom) : null;
+            projectsByIndex = in.readMap(Index::new, ProjectId::readFrom);
         } else {
-            projectsByIndex = null;
+            // Older nodes do not include the index-to-project map, so we leave it empty. This means all indices will be treated as if the
+            // project is unknown. This does not matter as the map is only used in multi-project clusters which will not have old nodes.
+            projectsByIndex = Map.of();
         }
     }
 
@@ -243,10 +244,7 @@ public void writeTo(StreamOutput out) throws IOException {
             out.writeMap(statsByIndex);
         }
         if (out.getTransportVersion().onOrAfter(TransportVersions.NODES_STATS_SUPPORTS_MULTI_PROJECT)) {
-            out.writeBoolean(projectsByIndex != null);
-            if (projectsByIndex != null) {
-                out.writeMap(projectsByIndex);
-            }
+            out.writeMap(projectsByIndex);
         }
     }
 
@@ -258,7 +256,7 @@ public boolean equals(Object o) {
         return stats.equals(that.stats)
             && statsByShard.equals(that.statsByShard)
             && statsByIndex.equals(that.statsByIndex)
-            && Objects.equals(projectsByIndex, that.projectsByIndex);
+            && projectsByIndex.equals(that.projectsByIndex);
     }
 
     @Override