query: suppress plan is not a logicalplan.Node errors

This failed cast is not really an error and it floods logs with message when using Ruler with gRPC queries.

Signed-off-by: Lukasz Mierzwa <lukasz@cloudflare.com>

Author: prymitive

pkg/api/query/querypb/plan.go

@@ -4,15 +4,14 @@
 package querypb
 
 import (
-	"github.com/pkg/errors"
 	"github.com/thanos-io/promql-engine/api"
 	"github.com/thanos-io/promql-engine/logicalplan"
 )
 
 func NewJSONEncodedPlan(plan api.RemoteQuery) (*QueryPlan, error) {
 	node, ok := plan.(logicalplan.Node)
 	if !ok {
-		return nil, errors.New("plan is not a logicalplan.Node")
+		return nil, nil
 	}
 	bytes, err := logicalplan.Marshal(node)
 	if err != nil {