Distinguish between continuous darkness / daylight

[susannahameyer]

https://github.com/nathan-osman/go-sunrise/blob/master/sunrise.go#L26-L28

This currently returns Time{} for sunrise and sunset when either sunrise or sunset never occurs. Is there a way to represent if the sun never sets vs. never rises to know whether there's darkness or daylight?

Example using latitude 78.22240149999999, longitude 15.651782 for Svalbard.

DateTime: 2024-06-12T23:21:00Z
Sunrise: 0001-01-01 00:00:00 +0000 UTC
Sunset: 0001-01-01 00:00:00 +0000 UTC

DateTime: 2024-12-12T23:21:00Z
Sunrise: 0001-01-01 00:00:00 +0000 UTC
Sunset: 0001-01-01 00:00:00 +0000 UTC

[elgohr]

Could return nil, but this would break the interface
See https://github.com/nathan-osman/go-sunrise/blob/master/sunrise_test.go#L39