clientv3: prevent nil pointer call in get method

Signed-off-by: Marcondes Viana <marju10@gmail.com>

client/v3/concurrency/stm.go

@@ -303,6 +303,10 @@ type stmSerializable struct {
 }
 
 func (s *stmSerializable) Get(keys ...string) string {
+	if len(keys) == 0 {
+		return ""
+	}
+
 	if wv := s.wset.get(keys...); wv != nil {
 		return wv.val
 	}

client/v3/concurrency/stm_test.go

@@ -0,0 +1,52 @@
+// Copyright 2023 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package concurrency
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGet(t *testing.T) {
+	tests := []struct {
+		name string
+		stm  *stmSerializable
+		in   []string
+		resp string
+	}{
+		{
+			name: "Empty keys returns empty string",
+			stm:  &stmSerializable{},
+			in:   []string{},
+			resp: "",
+		},
+		{
+			name: "Nil keys returns empty string",
+			stm:  &stmSerializable{},
+			in:   nil,
+			resp: "",
+		},
+	}
+
+	for _, test := range tests {
+		test := test
+		t.Run(test.name, func(t *testing.T) {
+			resp := test.stm.Get(test.in...)
+
+			assert.Equal(t, test.resp, resp)
+		})
+	}
+}