horizon: Properly check against the HA array being empty (#4152)

Because of the way strings.Split works, the returned array will
*always* have at least one element. If the string is empty, the
array will be [""], so we need to check [1] == "" instead of
checking a length of zero.

services/horizon/internal/flags.go

@@ -559,7 +559,7 @@ func ApplyFlags(config *Config, flags support.ConfigOptions, options ApplyOption
 
 		// config.HistoryArchiveURLs contains a single empty value when empty so using
 		// viper.GetString is easier.
-		if len(config.HistoryArchiveURLs) == 0 {
+		if len(config.HistoryArchiveURLs) == 1 && config.HistoryArchiveURLs[0] == "" {
 			return fmt.Errorf("--history-archive-urls must be set when --ingest is set")
 		}