Add small test for NothingColumn type

Author: slabko

ut/client_ut.cpp

@@ -434,6 +434,16 @@ TEST_P(ClientCase, Nullable) {
     EXPECT_EQ(sizeof(TEST_DATA) / sizeof(TEST_DATA[0]), row);
 }
 
+TEST_P(ClientCase, Nothing) {
+    client_->Select("SELECT NULL", [](const Block & block)
+        {
+            EXPECT_EQ(block.GetColumnCount(), 1UL);
+            EXPECT_EQ(block[0]->AsStrict<ColumnNothing>()->At(0), nullptr);
+            EXPECT_TRUE(block[0]->AsStrict<ColumnNullable>()->IsNull(0));
+            EXPECT_FALSE(block[0]->AsStrict<ColumnNullableT<ColumnInt8>>()->At(0).has_value());
+        });
+}
+
 TEST_P(ClientCase, Numbers) {
     try {
         size_t num = 0;