fix: NPE when reading BigQueryResultSet from empty tables (#3627)

* fix: NPE when reading BigQueryResultSet from empty tables

* Add unittests

Author: PhongChuong

google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/BigQueryResultImpl.java

@@ -113,6 +113,9 @@ private class BigQueryResultSet extends AbstractJdbcResultSet {
     @Override
     /*Advances the result set to the next row, returning false if no such row exists. Potentially blocking operation*/
     public boolean next() throws SQLException {
+      if (buffer == null) {
+        return false;
+      }
       if (hasReachedEnd) { // if end of stream is reached then we can simply return false
         return false;
       }

google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/Connection.java

@@ -140,7 +140,7 @@ ListenableFuture<ExecuteSelectResponse> executeSelectAsync(String sql)
    *   &#64;code
    *  ConnectionSettings connectionSettings =
    *        ConnectionSettings.newBuilder()
-   *            ..setUseReadAPI(true)
+   *            .setUseReadAPI(true)
    *            .build();
    *   Connection connection = bigquery.createConnection(connectionSettings);
    *     String selectQuery =

google-cloud-bigquery/src/test/java/com/google/cloud/bigquery/ConnectionImplTest.java

@@ -411,7 +411,7 @@ public void testLegacyQuerySinglePage() throws BigQuerySQLException {
 
   // calls executeSelect with a nonFast query where the query returns an empty result.
   @Test
-  public void testLegacyQuerySinglePageEmptyResults() throws BigQuerySQLException {
+  public void testLegacyQuerySinglePageEmptyResults() throws BigQuerySQLException, SQLException {
     ConnectionImpl connectionSpy = Mockito.spy(connection);
     com.google.api.services.bigquery.model.Job jobResponseMock =
         new com.google.api.services.bigquery.model.Job()
@@ -428,6 +428,10 @@ public void testLegacyQuerySinglePageEmptyResults() throws BigQuerySQLException
     BigQueryResult res = connectionSpy.executeSelect(SQL_QUERY);
     assertEquals(res.getTotalRows(), 0);
     assertEquals(QUERY_SCHEMA, res.getSchema());
+    assertEquals(
+        false,
+        res.getResultSet()
+            .next()); // Validates that NPE does not occur when reading from empty ResultSet.
     verify(bigqueryRpcMock, times(1))
         .createJobForQuery(any(com.google.api.services.bigquery.model.Job.class));
   }

google-cloud-bigquery/src/test/java/com/google/cloud/bigquery/it/ITBigQueryTest.java

@@ -3491,6 +3491,24 @@ public void testExecuteSelectDefaultConnectionSettings() throws SQLException {
     assertEquals(42, bigQueryResult.getTotalRows());
   }
 
+  @Test
+  public void testExecuteSelectReadApiEmptyResultSet() throws SQLException {
+    ConnectionSettings connectionSettings =
+        ConnectionSettings.newBuilder()
+            .setJobTimeoutMs(
+                Long.MAX_VALUE) // Force executeSelect to use ReadAPI instead of fast query.
+            .setUseReadAPI(true)
+            .setUseQueryCache(false)
+            .build();
+    Connection connection = bigquery.createConnection(connectionSettings);
+    String query = "SELECT TIMESTAMP '2022-01-24T23:54:25.095574Z' LIMIT 0";
+    BigQueryResult bigQueryResult = connection.executeSelect(query);
+
+    ResultSet rs = bigQueryResult.getResultSet();
+    assertThat(rs.next()).isFalse();
+    assertThat(bigQueryResult.getTotalRows()).isEqualTo(0);
+  }
+
   @Test
   public void testExecuteSelectWithCredentials() throws SQLException {
     // This test validate that executeSelect uses the same credential provided by the BigQuery