Fix querycoord panic when stop nil conn of querynoClient (#7346)

Signed-off-by: xige-16 <xi.ge@zilliz.com>

internal/distributed/querynode/client/client.go

@@ -127,7 +127,11 @@ func (c *Client) Start() error {
 }
 
 func (c *Client) Stop() error {
-	return c.conn.Close()
+	c.cancel()
+	if c.conn != nil {
+		c.conn.Close()
+	}
+	return nil
 }
 
 // Register dummy

internal/distributed/querynode/client/client_test.go

@@ -0,0 +1,15 @@
+package grpcquerynodeclient
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestNilClient(t *testing.T) {
+	client, err := NewClient(context.Background(), "test")
+	assert.Nil(t, err)
+	err = client.Stop()
+	assert.Nil(t, err)
+}