fix: default value of datasource when no environment variables

[yangliyl]

postgres_exporter/cmd/postgres_exporter/main.go

Lines 88 to 97 in 92bdb87

	dsn, err := getDataSources()
	if err != nil {
	level.Error(logger).Log("msg", "Failed reading data sources", "err", err.Error())
	os.Exit(1)
	}
	if len(dsn) == 0 {
	level.Error(logger).Log("msg", "Couldn't find environment variables describing the datasource to use")
	os.Exit(1)
	}

default value of datasource when no environment variables
[postgresql://:@]

check is always false