Prevent using custom enpoints if one is not defined

The AWS endpoint sanititazion was inadvertantly being applied all
the time, causing configurations not using custom endpoints to be
forced into using a custom endpoint instead of AWS.

lib/events/dynamoevents/dynamoevents.go

@@ -208,7 +208,9 @@ func (cfg *Config) CheckAndSetDefaults() error {
 		cfg.UIDGenerator = utils.NewRealUID()
 	}
 
-	cfg.Endpoint = endpoint.CreateURI(cfg.Endpoint, cfg.Insecure)
+	if cfg.Endpoint != "" {
+		cfg.Endpoint = endpoint.CreateURI(cfg.Endpoint, cfg.Insecure)
+	}
 
 	return nil
 }

lib/events/s3sessions/s3handler.go

@@ -157,7 +157,9 @@ func (s *Config) CheckAndSetDefaults() error {
 		return trace.BadParameter("missing parameter Bucket")
 	}
 
-	s.Endpoint = endpoint.CreateURI(s.Endpoint, s.Insecure)
+	if s.Endpoint != "" {
+		s.Endpoint = endpoint.CreateURI(s.Endpoint, s.Insecure)
+	}
 
 	return nil
 }

lib/utils/aws/endpoint/endpoint.go

@@ -33,7 +33,7 @@ var schemeRegex = regexp.MustCompile("^([^:]+)://")
 // a custom service endpoint, this performs applies the same
 // behavior that the legacy sdk did.
 func CreateURI(endpoint string, insecure bool) string {
-	if !schemeRegex.MatchString(endpoint) {
+	if endpoint != "" && !schemeRegex.MatchString(endpoint) {
 		scheme := "https"
 		if insecure {
 			scheme = "http"

lib/utils/aws/endpoint/endpoint_test.go

@@ -31,6 +31,9 @@ func TestCreateURI(t *testing.T) {
 		insecure bool
 		expected string
 	}{
+		{
+			name: "empty endpoint",
+		},
 		{
 			name:     "valid endpoint",
 			endpoint: "https://test.example.com",