fix(GraphQL): fixes panic in update mutation without set & remove (#6073

) (#6160)

FIxes GRAPHQL-589.
See [Discuss](https://discuss.dgraph.io/t/panic-when-update-with-filter-but-without-set-remove/8623) for more details.

(cherry picked from commit d82ecb4)

graphql/e2e/common/common.go

@@ -313,6 +313,7 @@ func RunAll(t *testing.T) {
 	t.Run("mutations have extensions", mutationsHaveExtensions)
 	t.Run("alias works for mutations", mutationsWithAlias)
 	t.Run("three level deep", threeLevelDeepMutation)
+	t.Run("update mutation without set & remove", updateMutationWithoutSetRemove)
 
 	// error tests
 	t.Run("graphql completion on", graphQLCompletionOn)

graphql/e2e/common/mutation.go

@@ -2908,7 +2908,6 @@ func threeLevelDeepMutation(t *testing.T) {
 	}
 
 	gqlResponse := postExecutor(t, graphqlURL, addStudentParams)
-	fmt.Println(string(gqlResponse.Data))
 	RequireNoGQLErrors(t, gqlResponse)
 
 	var actualResult struct {
@@ -3175,3 +3174,32 @@ func mutationsWithAlias(t *testing.T) {
 
 	require.JSONEq(t, multiMutationExpected, string(gqlResponse.Data))
 }
+
+func updateMutationWithoutSetRemove(t *testing.T) {
+	country := addCountry(t, postExecutor)
+
+	updateCountryParams := &GraphQLParams{
+		Query: `mutation updateCountry($id: ID!){
+			updateCountry(input: {filter: {id: [$id]}}) {
+				numUids
+				country {
+					id
+					name
+				}
+			}
+		}`,
+		Variables: map[string]interface{}{"id": country.ID},
+	}
+	gqlResponse := updateCountryParams.ExecuteAsPost(t, graphqlURL)
+	RequireNoGQLErrors(t, gqlResponse)
+
+	require.JSONEq(t, `{
+		"updateCountry": {
+			"numUids": 0,
+			"country": []
+    	}
+	}`, string(gqlResponse.Data))
+
+	// cleanup
+	deleteCountry(t, map[string]interface{}{"id": []string{country.ID}}, 1, nil)
+}

graphql/resolve/mutation.go

@@ -245,6 +245,18 @@ func (mr *dgraphResolver) rewriteAndExecute(ctx context.Context,
 		return emptyResult(schema.GQLWrapf(err, "couldn't rewrite mutation %s", mutation.Name())),
 			resolverFailed
 	}
+	if len(upserts) == 0 {
+		return &Resolved{
+			Data: map[string]interface{}{
+				mutation.Name(): map[string]interface{}{
+					schema.NumUid:                0,
+					mutation.QueryField().Name(): nil,
+				}},
+			Field:      mutation,
+			Err:        nil,
+			Extensions: ext,
+		}, resolverSucceeded
+	}
 
 	result := make(map[string]interface{})
 	req := &dgoapi.Request{}