Allow updating fields to null

data/test/vtgate/dml_cases.txt

@@ -1512,3 +1512,24 @@
     "Query": "delete from unsharded where col = (select id from unsharded_a where id = unsharded.col)"
   }
 }
+
+# update vindex value to null
+"update user set name = null where id = 1"
+{
+  "Original": "update user set name = null where id = 1",
+  "Instructions": {
+    "Opcode": "UpdateEqual",
+    "Keyspace": {
+      "Name": "user",
+      "Sharded": true
+    },
+    "Query": "update user set name = null where id = 1",
+    "Vindex": "user_index",
+    "Values": [1],
+    "ChangedVindexValues": {
+      "name_user_map": [null]
+    },
+    "Table": "user",
+    "OwnedVindexQuery": "select Name, Costly from user where id = 1 for update"
+  }
+}

go/vt/vtgate/planbuilder/update.go

@@ -172,7 +172,7 @@ func generateUpdateSubquery(upd *sqlparser.Update, table *vindexes.Table) string
 // it's holding. At the moment it only supports: StrVal, HexVal, IntVal, ValArg.
 // If a complex expression is provided (e.g set name = name + 1), the update will be rejected.
 func extractValueFromUpdate(upd *sqlparser.UpdateExpr) (pv sqltypes.PlanValue, err error) {
-	if !sqlparser.IsValue(upd.Expr) {
+	if !sqlparser.IsValue(upd.Expr) && !sqlparser.IsNull(upd.Expr) {
 		err := vterrors.Errorf(vtrpcpb.Code_UNIMPLEMENTED, "unsupported: Only values are supported. Invalid update on column: %v", upd.Name.Name)
 		return sqltypes.PlanValue{}, err
 	}