Don't return empty routing key when partition key is unbound

DefaultBoundStatement#getRoutingKey has logic to infer the
routing key when no one has explicitly called setRoutingKey
or otherwise set the routing key on the statement.
It however doesn't check for cases where nothing has been
bound yet on the statement.
This causes more problems if the user decides to get a
BoundStatementBuilder from the PreparedStatement, set some
fields on it, and then copy it by constructing new
BoundStatementBuilder objects with the BoundStatement as a
parameter, since the empty ByteBuffer gets copied to all
bound statements, resulting in all requests being targeted
to the same Cassandra node in a token-aware load balancing
policy.

patch by Ammar Khaku; reviewed by Andy Tolbert, and Bret McGuire
reference: #1620

Author: akhaku

core/src/main/java/com/datastax/oss/driver/internal/core/cql/DefaultBoundStatement.java

@@ -360,7 +360,8 @@ public ByteBuffer getRoutingKey() {
       if (indices.isEmpty()) {
         return null;
       } else if (indices.size() == 1) {
-        return getBytesUnsafe(indices.get(0));
+        int index = indices.get(0);
+        return isSet(index) ? getBytesUnsafe(index) : null;
       } else {
         ByteBuffer[] components = new ByteBuffer[indices.size()];
         for (int i = 0; i < components.length; i++) {

integration-tests/src/test/java/com/datastax/oss/driver/core/cql/PreparedStatementIT.java

@@ -527,6 +527,25 @@ private void should_infer_routing_information_when_partition_key_is_bound(String
     assertThat(tokenFactory.hash(boundStatement.getRoutingKey())).isEqualTo(expectedToken);
   }
 
+  @Test
+  public void should_return_null_routing_information_when_single_partition_key_is_unbound() {
+    should_return_null_routing_information_when_single_partition_key_is_unbound(
+        "SELECT a FROM prepared_statement_test WHERE a = ?");
+    should_return_null_routing_information_when_single_partition_key_is_unbound(
+        "INSERT INTO prepared_statement_test (a) VALUES (?)");
+    should_return_null_routing_information_when_single_partition_key_is_unbound(
+        "UPDATE prepared_statement_test SET b = 1 WHERE a = ?");
+    should_return_null_routing_information_when_single_partition_key_is_unbound(
+        "DELETE FROM prepared_statement_test WHERE a = ?");
+  }
+
+  private void should_return_null_routing_information_when_single_partition_key_is_unbound(
+      String queryString) {
+    CqlSession session = sessionRule.session();
+    BoundStatement boundStatement = session.prepare(queryString).bind();
+    assertThat(boundStatement.getRoutingKey()).isNull();
+  }
+
   private static Iterable<Row> firstPageOf(CompletionStage<AsyncResultSet> stage) {
     return CompletableFutures.getUninterruptibly(stage).currentPage();
   }