Running `hours_sunrise_sunset` plot fails on locations where the sun doesn't set

#### Description:

When running `hours_sunrise_sunset` on a location where any of the sunset times cannot be retrieved, the plot fails and doesn not output anything (due to a.normalize() not working when sunset/sunrise times are null).

#### Steps to reproduce:

Run `hours_sunrise_sunset` on a location where the sun does not set (this includes twilight zones), e.g. Aberdeen.

#### Expected behaviour:

A plot that shows the hours sunrise sunset where in the middle, the block spans the height of the plot.

#### Test file(s):

using a suitable location
```
hours_sunrise_sunset(location=location)
```

#### Additional Comments

This also appears to affect hours_sunlight as well, but doesn't show errors and plots the following: