add null table exception

Author: aasthabharill

v2/datastream-to-spanner/src/main/java/com/google/cloud/teleport/v2/templates/datastream/ChangeEventContext.java

@@ -78,6 +78,10 @@ protected ChangeEventContext(
     this.changeEvent = changeEvent;
     this.dataTable = changeEvent.get(DatastreamConstants.EVENT_TABLE_NAME_KEY).asText();
     Table table = ddl.table(this.dataTable);
+    if (table == null) {
+      throw new DroppedTableException(
+          "Table from change event does not exist in Spanner. table=" + this.dataTable);
+    }
     Set<String> existingPrimaryKeyColumnNames =
         table.primaryKeys().stream().map(k -> k.name()).collect(Collectors.toSet());