Fix nil pointer exception when no object storage config is defined

This is a regression that was caused if the `[object_storage]` config
section were omitted.

changelogs/unreleased/sh-fix-nil-object-storage-creds.yml

+---
+title: Fix nil pointer exception when no object storage config is defined
+merge_request: 565
+author:
+type: fixed

internal/config/config.go

@@ -111,7 +111,11 @@ func LoadConfig(filename string) (*Config, error) {
 
 func (c *Config) RegisterGoCloudURLOpeners() error {
 	c.ObjectStorageConfig.URLMux = new(blob.URLMux)
+
 	creds := c.ObjectStorageCredentials
+	if creds == nil {
+		return nil
+	}
 
 	if strings.EqualFold(creds.Provider, "AzureRM") && creds.AzureCredentials.AccountName != "" && creds.AzureCredentials.AccountKey != "" {
 		accountName := azureblob.AccountName(creds.AzureCredentials.AccountName)

internal/config/config_test.go

@@ -8,6 +8,18 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+func TestLoadEmptyConfig(t *testing.T) {
+	config := ``
+
+	tmpFile, cfg := loadTempConfig(t, config)
+	defer os.Remove(tmpFile.Name())
+
+	require.Nil(t, cfg.ObjectStorageCredentials)
+
+	err := cfg.RegisterGoCloudURLOpeners()
+	require.NoError(t, err)
+}
+
 func TestLoadObjectStorageConfig(t *testing.T) {
 	config := `
 [object_storage]