Fix metadata events for case when tablet map is empty (#507)

Tablet map could be empty, we need to address these cases.

Author: dkropachev

core/src/main/java/com/datastax/oss/driver/internal/core/metadata/AddTabletRefresh.java

@@ -20,7 +20,9 @@ public AddTabletRefresh(CqlIdentifier keyspace, CqlIdentifier table, Tablet tabl
   @Override
   public Result compute(
       DefaultMetadata oldMetadata, boolean tokenMapEnabled, InternalDriverContext context) {
-    oldMetadata.tabletMap.addTablet(keyspace, table, tablet);
+    if (oldMetadata.tabletMap != null) {
+      oldMetadata.tabletMap.addTablet(keyspace, table, tablet);
+    }
     return new Result(oldMetadata);
   }
 }

core/src/main/java/com/datastax/oss/driver/internal/core/metadata/RemoveNodeRefresh.java

@@ -67,7 +67,9 @@ public Result compute(
       return new Result(oldMetadata);
     } else {
       LOG.debug("[{}] Removing node {}", logPrefix, removedNode);
-      oldMetadata.tabletMap.removeByNode(removedNode);
+      if (oldMetadata.tabletMap != null) {
+        oldMetadata.tabletMap.removeByNode(removedNode);
+      }
       return new Result(
           oldMetadata.withNodes(newNodesBuilder.build(), tokenMapEnabled, false, null, context),
           ImmutableList.of(NodeStateEvent.removed((DefaultNode) removedNode)));