Swap nil PerformanceData check for length check

[atc0005]

Current check:

go-nagios/nagios.go

Lines 385 to 389 in b399b44

	func (es *ExitState) AddPerfData(skipValidate bool, pd ...PerformanceData) error {
	if pd == nil {
	return fmt.Errorf("no performance data provided")
	}

Use length check instead of nil slice check. I don't know whether it's legitimate to pass in an initialized but empty slice, but a length check would catch that scenario in addition to an actual nil slice.