fix: JSON nulls were inserted as STRINGs and rejected (#952)

Null values for JSON columns were inserted with type STRING, and then
rejected by Cloud Spanner. This change also overrides the setNull
methods, so these also explicitly state that the null value is a JSON.

google-cloud-spanner-hibernate-dialect/src/main/java/com/google/cloud/spanner/hibernate/SpannerDialect.java

@@ -101,6 +101,18 @@ protected void doBind(CallableStatement st, X value, String name, WrapperOptions
               value, getJavaType(), options);
           st.setObject(name, json, JsonType.VENDOR_TYPE_NUMBER);
         }
+
+        @Override
+        protected void doBindNull(PreparedStatement st, int index, WrapperOptions options)
+            throws SQLException {
+          st.setNull(index, JsonType.VENDOR_TYPE_NUMBER);
+        }
+
+        @Override
+        protected void doBindNull(CallableStatement st, String name, WrapperOptions options)
+            throws SQLException {
+          st.setNull(name, JsonType.VENDOR_TYPE_NUMBER);
+        }
       };
     }
   }

google-cloud-spanner-hibernate-dialect/src/test/java/com/google/cloud/spanner/hibernate/it/SampleModelIT.java

@@ -259,6 +259,20 @@ public void testSaveVenue() {
     }
   }
 
+  @Test
+  public void testSaveVenueWithNullDescription() {
+    try (Session session = sessionFactory.openSession()) {
+      final Transaction transaction = session.beginTransaction();
+      Venue venue = new Venue("Venue 2", /* description = */ null);
+      session.persist(venue);
+      transaction.commit();
+
+      session.refresh(venue);
+      assertEquals("Venue 2", venue.getName());
+      assertNull(venue.getDescription());
+    }
+  }
+
   @Test
   public void testSaveConcert() {
     try (Session session = sessionFactory.openSession()) {